Implement correct printing for Ptyp_package as core_type.

[IwanKaramazow]

Fixes #2122

Also adds support for less parens in the context of a Ptyp_constr/Ptyp_tuple:

/* before */
type t = ref((module Console));

/* after */
type t = ref(module Console);

[hcarty]

Does this cover record fields like type t = { m: (module M) }; too?

Thank you for working on this! I want to use Reason's master branch for the improved merlin support but this bug breaks a lot of code for me vs 3.2.0.

[IwanKaramazow]

@hcarty added a test case, I'll update the PR to allow parsing without parens in that case.