Merge: Vim omnifunc diffentiate between types, class declaration and instanciation

The main improvement is the knowledge of constructors. They will be suggested along with their class after a `new`, ex: `new Arr`, it will autocomplete to `Array[E]`, `Array[E].with_items`, etc. It will also show their corresponding documentation.

Also detect the keywords `isa` and `as`.

Pull-Request: #1156
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>