Allow a module to implement an interface

It would be useful when a module can implement an interface using the `implements` keyword. Syntax: `module MyModule implements MyInterface { ... }`.

Example:

``` typescript
interface Showable {
show(): void;
}
function addShowable(showable: Showable) {

}

// This works:
module Login {
export function show() {
document.getElementById('login').style.display = 'block';
}
}
addShowable(Login);

// This doesn't work (yet?)
module Menu implements Showable {
export function show() {
document.getElementById('menu').style.display = 'block';
}
}
addShowable(Menu);
```

The issue names no source file, test, or compiler entry point. Start by locating handling for module declarations and interface compatibility, then add coverage for `module Menu implements Showable` and confirm the existing `addShowable(Menu)` example type-checks.

Written by the indexing model from the issue text.