Create a new input syntax for function.

[DavidFangWJ]

Motivation

Add a more compact syntax to function, such that

let f(x : A, y: B): C = 
  e

should behave the same as below:

let f : (A, B) -> C = 
  fun (x, y) -> e

This new function syntax is more analogous to other programming languages with strong data types, and is therefore easier to learn.

Implementation Strategy

I believe that it will mostly be a modification of Form.re.

• [ ] Add new syntax expression
• [ ] Make sure that its further processing is identical to existing function syntax