Parse the (!) type operator and allow type operators in existential context

Summary:
Improve the way `(!)`, `(~)`, and other type operators are handled in the parser,
fixing two issues at once:

1. `(!)` can now be used as a type operator
   that respects fixity and precedence (#15457)
2. Existential context of a data constructor
   no longer needs parentheses (#15675)

In addition to that, with this patch it is now trivial to adjust precedence of
the `{-# UNPACK #-}` pragma, as suggested in
https://ghc.haskell.org/trac/ghc/ticket/14761#comment:7

There was a small change to API Annotations. Before this patch, `(~)` was a
strange special case that produced an annotation unlike any other type
operator. After this patch, when `(~)` or `(!)` are used to specify strictness they
produce AnnTilde and AnnBang annotations respectively, and when they are used
as type operators, they produce no annotations.

Test Plan: Validate

Reviewers: simonpj, bgamari, alanz, RyanGlScott

Reviewed By: RyanGlScott

Subscribers: RyanGlScott, rwbarton, mpickering, carter

GHC Trac Issues: #15457, #15675

Differential Revision: https://phabricator.haskell.org/D5180

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