Unify GADT syntax

See the discussion here: https://github.com/facebook/reason/issues/491#issuecomment-222324029

We can make GADTs less scary, by making them a natural syntactic extension of regular variant declarations.

One proposal is as follows, but is more verbose, so if you have better ideas, please discuss here:

If regular variant definitions were as follows:

type something = 
  | Constructor (int, int) : something
  | Another (list int) : something;

Then creating a GADT is not such a huge step, and the syntax helps to explain what GADTs are in the first place.

type something 'a = 
  | Constructor (int, int) : something int
  | Another (list int) : something unit;

The only problem is that (as the first example shows), in the case of regular variant definitions, there is a lot of redundancy. I'm hoping someone has a good proposed solution for this.

Start with the linked discussion at https://github.com/facebook/reason/issues/491#issuecomment-222324029 and compare the regular variant and GADT syntax examples in this issue. No implementation files or tests are named. The work is done when a syntax proposal is agreed on and its required implementation and tests are identified.