Value objects (#293)

This PR introduces `Literal::Value` objects. Literal::Values are
designed to wrap primitives (though they can wrap any object) in a
branded object allowing for more precise type-matching.

```ruby
UserID = Literal::Value(Integer)

my_user_id = UserID.new(123)
```

### Constraints

You can pass constraints, same as `_Constraint`

```ruby
Username = Literal::Value(String, length: 3..15)
```

### Coercion

Literal value classes can be coerced into a coercion by default

```ruby
prop :user, UserID, &UserID
```

### Delegation

You can delegate to underlying methods

```ruby
Username = Literal::Value(String) do
delegate :length
end
```

Literal will default to delegating to basic coercion methods. For
example, if your value is a String, it will delegate `to_s` and `to_str`
to the underlying value by default. Otherwise, delegation is explicit.

Only delegate what you need. Constraining the interface allows for more
flexibility later on. If you want to replace a value object with a
regular object, you won’t need to re-implement the entire interface of a
primitive.

### Customisation

```ruby
Name = Literal::Value(String) do
def first
@value.split(" ").first
end
end
```

### Freezing

Literal value classes and literal value objects are frozen by default,
though their underlying values are not automatically frozen.