Meeseeks.Parser is private, hiding module types in documentation

[mischov]

Reported by @davidsulc

Currently, the Meeseeks.Parser module is private (@moduledoc false) which means that it's not possible for users to determine the proper parameters to (e.g.) Meeseeks.parse/2 by looking at the docs (as they refer to an undocumented module)

[mischov]

Various improvements will need to be made to Meeseeks.Parser before it can be made public.