Reason: [GoodFirstTask] Fix formatting of GADTs

[jordwalke]

I was experimenting with some new Reason React JSX representations and I noticed that in one of the recent syntax refactorings, formatting of GADTs became very ugly:

type jsx('nature) =
  | Empty: jsx(empty)
  | Cons(elem(('s1, 'a1), 'sub1), jsx('tl)): jsx(
                                                   (('s1, 'a1) => 'sub1, 'tl),
                                                 );

This is a good first task if anyone is interested in learning the code base.

[jordwalke]

To new people looking to get started:

• Claim this task if you want to work on it. Ask me or others in Discord for help if necessary.
• Clone the Reason repo, and get it building on master. That shouldn't be hard.
• Start by finding out what AST shape is produced by that code sample above, by running cat fileThatHasThatSample.re | refmt --parse re --print ast. (Or if you're using the esy workflow for developing, prefix the call to refmt ... like esy refmt ....
• Start tweaking the formatting code in ./src/reason-parser/reason_pprint_ast.ml

[mrandri19]

I'll have a look at this