[new release] links, links-postgresql, links-sqlite3 and links-mysql (0.9.7)

CHANGES:

This release includes minor bug fixes, improvements, and a breaking
change.

* **Breaking change**: The Links standard library has adopted `Maybe`
as it optional type. Previously, the standard library made
inconsistent use of both `Option` and `Maybe` types. As a
consequence of this change, every instance of `Option` has been
turned into a `Maybe` type.
* Uniform paths: The file paths used by Links internally are now
uniformly computed at compile time from the base of the current OPAM
switch.
* Support for record extension in the presence of temporal projections.
* It is now to use `default` as a setting value in the REPL.
* Fixed an assertion failure in the relational lens code base.
* Effect aliases: It is now possible to define aliases for effect
signatures, e.g.

```links
effectname State(s, e::Eff) = {Get:s, Put:(s) -> () |e};
```

defines an alias named `State` for an effect row with two operations
`Get`, and `Put`. This alias is analogous to aliases introduced via
`typename`, except that `effectname` aliases may be used in places
where an effect row is expected, e.g.

```links
sig f : () ~State(Int, {})~> ()
```

is equivalent to

```links
sig f : () {Get:Int, Put:(Int) -> ()}~> ()
```

* Client/server annotations are now permitted on local and anonymous
functions.
* Fixed a bug where remote calls involving a client closure would
cause the server side to crash.