Simplify grammar & syntax (parens) of first class modules

[IwanKaramazow]

Fixes #530
Fixes #1715

Goals

1. Simplify menhir grammar by over-parsing
2. Allow first-class modules with less parentheses

Result

• Simpler constraints on let-bindings, uppercase automatically implies a Ptyp_package

* Less parentheses
```reason
/* before */
let f = ((module Add : S.Z), x) => Add.add(x);
module Modifier = (val (Db.Hashtbl.create (): (module Db.Sig with type t = Mods.t)));
let thing: (module Thing) = (module MyModule);

/* after */
let f = (module Add : S.Z, x) => Add.add(x);
module Modifier = (val Db.Hashtbl.create (): Db.Sig with type t = Mods.t);
let thing: module Thing = (module MyModule);

• let module syntax will need to be deprecated, because of too much conflicts.
  Update: let module-syntax stays as is

[chenglou]

let module is long deprecated and refmting to module Foo = .... So I think it's an ok time to get rid of it

[IwanKaramazow]

@chenglou this is ready.
TLDR

• simplify grammar where possible
• less parens for first class modules (parsing + printing)
• no deprecations, let module stays as is.

[chenglou]

For let thing: Thing = (module MyModule), is it possible to still not accept it? Only let thing: module Thing = (module MyModule)?

I'm not too sure that we're at a stage where folks all use refmt yet

[IwanKaramazow]

Sure, no problem, I'll restructure the grammar.

Update: done ✅

[chenglou]

Do we not have a similar utility already? Genuine question. We should have a single one that throws a single type of exception

[chenglou]

So apparently @jaredly still uses let module. Otherwise I think we can kill let module?

[jaredly]

I super like let module because it's one less difference to memorize between statement-level syntax and expression-level syntax.
If we could make module X = Y work at the expression level, I'd be fine with removing let module everywhere

[hcarty]

@jaredly Where does module X = Y not currently work in expressions? I thought that worked anywhere let module worked.

Edit: "not", not "it"

[jaredly]

aha I guess it does work :P ok, I'd be ok with it going away.

…

On Sat, Jun 2, 2018 at 5:05 PM Hezekiah M. Carty ***@***.***> wrote: @jaredly <https://github.com/jaredly> Where does module X = Y it currently work in expressions? I thought that worked anywhere let module worked. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#1949 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAG2KkmazaDq37dI8DwSO1CgckBmdkzvks5t4xpRgaJpZM4ULHf6> .

[chenglou]

We’ll kill it later. Keeping it for another version at least for now.

Thanks!