Fix signature-help with type aliases

[lyrm]

Fixes issue #1927.

What changes

signature help now expands types once to decide whether a value is a function:

type t = int -> unit
let f : t = fun _ -> ()
let _ = f  (* before: nothing; after: f : int -> unit *)

Same when there is an alias exposed mid-call: if the call supplies more arguments than the visible arrows allow, the alias is expanded.

type t = int -> unit
let f (_ : int) : t = …
let _ = f 1 2  (* f : int -> int -> unit *)

What have not changed

type t = int * int
let f (_ : t) = ()         
let _ = f  (* f : t -> unit, not f : int * int -> unit *)

type t = int -> unit
let f (_ : int) : t = unit 
let _ = f  (* f : int -> t` *)

What is still broken? (or is it on purpose?)

let f ~x = x + 1     
let _ = f  (* nothing *)

Even so

let f ~x = x + 1     
let _ = f  ~  (* f: x:int -> int *)

and

type t = x:int -> int
let f : t = fun ~x -> x + 1     
let _ = f  ~  (* f: x:int -> int *)

both work.

[xvw]

The implementation seems reasonnable (and the test suite convincing). Thanks a lot.