Add support for destruct

[Khady]

Merlin is able to destruct a value to create a pattern matching on it. It would be nice to support this feature

Example:

let () =
  let a = Some 1 in
  a<CURSOR>

After calling destruct becomes

let () =
  let a = Some 1 in
  (match a with | None  -> (??) | Some _ -> (??))

From merlin documentation:

["case","analysis","from",position,"to",position]

Try to destruct patterns in the specified range. It returns a value with the shape [{"start": position, "end": position}, content]. The editor is expected to replace content between start and end by content.