Hack: Allow extending shapes

We're starting to move towards using shapes as option arguments and are having to duplicate most of the declaration between subclasses, which leads to problems when changing the base class.

Contrived example:

``` hack
int,
'b' => ?string,
);

type BOptions = shape(
'a' => int,
'b' => ?string,
'c' => (function():void),
);

class A {
public function __construct(protected AOptions $options) {}
}

class B extends A {
protected (function():void) $c;

public function __construct(BOptions $options) {
$this->c = $options['c'];
parent::__construct($options);
}
}
```

Being able to have `BOptions` extend `AOptions` would reduce the copy pasta and chance of errors. Something like:

``` hack
type BOptions = shape(
'c' => (function():void),
) extends AOptions;
```