[new release] links, links-postgresql, links-sqlite3 and links-mysql (0.9.4)

CHANGES:

## Queries mixing set and bag semantics
Links now provides experimental support for SQL queries mixing set and bag semantics.

When the `mixing_norm=on` flag is added to the configuration file, or when a query is defined in a `query mixing { ... }` block, Links will use a new query evaluator, allowing the programmer to call deduplication functions (`dedup` and `distinct`) within database queries. These are handled with set-based SQL statements `select distinct / union`, in addition to the usual bag-based `select / union all`.

# will run on the DB as "select distinct e.dept as dept from employees"
query mixing {
dedup(for (e <-- employees) [(dept = e.dept)])
}

Queries mixing set and bag semantics may, in some cases, require the use of the SQL:1999 keyword `lateral`; Links implements an optional query transformation to produce queries that do not use `lateral` (allowing the use of older DBMSs): this behaviour is enabled by using `query delat` in place of `query mixing`.

Further information on this feature is provided in the [Links GitHub wiki](https://github.com/links-lang/links/wiki/Deduplication-in-database-queries).

## DateTime type
Links now includes a primitive type, `DateTime`, for dates and times.
This is a *breaking change* from previous versions, where `dateToInt`
and `intToDate` operated on a record.

The primitive type allows us to better timezones, and also allows us
to work seamlessly with timestamps in the database.

Obtain a DateTime via:

* the `now()` function to get a timestamp for the current local time
* Using `parseDate` on an ISO-formatted string (e.g., `parseDate("26-07-2021 14:26:00+1")`)
* A `DateTime` field in the database
* `intToDate(X)` where `X` is a UNIX timestamp
* `beginningOfTime` and `forever`, which are special timestamps guaranteed to be less than (resp. greater than) all other timestamps

Project fields out of the type:

* utcYear, utcMonth, utcDay, utcHours, utcMinutes, utcSeconds, utcMilliseconds projects the given field in the UTC time zone
* localYear, localMonth, localDay, localHours, localMinutes, localSeconds, localMilliseconds projects the given field in the local time zone
* dateYear, dateMonth, dateDay, dateHours, dateMinutes, dateSeconds, dateMilliseconds projects the given field in a given timezone (e.g., to project the hours field of a DateTime dt in BST, one would write `dateHours(dt, 1)`, where 1 is the timezone offset.

You can also print out the `DateTime` using `show` (which is an alias of
`showLocal`) and `showUTC`.

`DateTime`s are comparable as normal.

Due to limitations of the underlying library, the minimum timezone granularity is one hour. Unfortunately, this means we can't handle Indian timezones, for example.

## New surface syntax

### Presence type arguments

**Breaking change**: New syntax has been added to support type arguments of kind `Presence`. Here are some example of the syntax:

```links
typename T(p::Presence) = (foo{p});

(foo=4200) : T({:Int}) # Present with type Int
() : T({-}) # Absent
(foo=4200) : T({%}) # Unnamed flexible variable
(foo=true) : T({%p}) # Named flexible variable
fun(r : T({_})) { () } # Anonymous presence variable
fun(r : T({p})) { r : T({p}) } # Named presence variable
```

The syntactic sugar for effect and record fields which lets one omit the `()` has been removed in order to resolve the otherwise ambiguity between the presence type argument `{wild}` from the row type argument `{wild}`. Note however that it is still possible to omit the `()` for variant fields.

### Mono restriction

It is now possible to annotate type variables with `Mono` restriction, e.g. `sig id : (a::(Any,Mono)) -> a::(Any,Mono)`.

### Recursive rows

* Effect variables can be recursive, e.g. `{ |(mu a.F:(() { |a}-> ()) {}-> b|c)}`.
* **Breaking change**: Recursive rows are no longer restricted to variant syntax, i.e. separating fields using `|`. Recursive record and effects rows separate fields using `,` now.
* Recursive variants with no directly exposed fields no longer require the vertical bar separating fields from the row variable, e.g. `[|(mu a. Foo)|]` is equivalent to `[| |(mu a . Foo)|]`.

## Roundtrip: New pretty printer for types

This version of Links introduces a new pretty printer for types, called Roundtrip. This fixes various round-tripping issues.

The Roundtrip printer is now active by default. The old printer is still present.

The printer(s) to be used can be selected using the setting `types_pretty_printer_engine`, with the following values:

* `roundtrip`: the new printer
* `old`: the original printer
* `derived`: no pretty printing - prints the OCaml representation of the types

Note that one can select multiple printers at once, for comparison; this is done by separating printer names by commas, e.g.:

```links
@set types_pretty_printer_engine "roundtrip,old";
```

## Effect Syntactic Sugar

This version implements enhanced syntactic sugar for effects. The changes influence both the Roundtrip printer (see above) and the desugaring passes (between parsing and typechecking).

(*Note: Most of effect sugar, and in particular the changes introduced in this version, requires the `effect_sugar` setting to be `true`.*)

There is a new setting `effect_sugar_policy` which allows one to set which components of effect sugar to use. The available options (with shortcuts for convencience) are:

* `presence_omit` [shotcut `pres`]: omit presence polymorphic operations within effect rows
* `alias_omit` [shortcut `alias`]: hide empty (and emptied using `pres`) shared effect rows in the last argument of aliases
* `arrows_show_implicit_effect_variable` [shortcut `show_implicit`]: display the imlicit shared effect on arrows
* `arrows_curried_hide_fresh` [shortcut `chf`]: in curried functions, argument collection arrows are assumed to have fresh effects and these are hidden
* `contract_operation_arrows` [shortcut `contract`]: contract operation arrows: `E:() {}-> a` to `E:a` and `E:(a) {}-> b` to `E:(a) -> b`
* `open_default` [shortcut `open`]: effect rows are open by default, closed with syntax `{ | .}`
* `final_arrow_shares_with_alias` [shortcut `final_arrow`]: final arrow and a following type alias may share implicit effects
* `all_implicit_arrows_share` [shortcut `all_arrows`]: all arrows with implicit effect vars will be unified, an experimental setting

Multiple of these can be selected, separated by commas, e.g.:

```links
@set effect_sugar_policy "pres,alias,contract";
```

A version of the above is also available by entering `@help effect_sugar_policy;` in Links.

These changes are explained in more depth and with examples in [Links GitHub Wiki/Effect Sugar](https://github.com/links-lang/links/wiki/Effect-Sugar).

## Other fixes / Miscellaneous

* Relational lenses are now enabled by default.
* Fixed a bug where the REPL would unconditionally print a stacktrace for unknown directives.
* Fixed a bug where deeply nested JSON literals would cause the client to stack overflow.
* Fixed a bug where big server side values would cause the client to stack overflow.
* Fixed JavaScript compilation of top-level anonymous functions.
* Fixed a bug where the server would inadvertently respond with response `500` following the (successful) termination of a server side process.
* The body of an escape expression has been made more permissive (grammatically) as it can now be any expression.