Make forall a keyword

At the moment, forall is a keyword at the type level but an identifier at the term level:

forall = undefined -- accepted by GHC today

GHC Proposal #281 makes forall a keyword at both levels.

Since this is a breaking change, the first step was to add a warning (see #20609 (closed)). This was done in dfc4093c.

Main.hs:5:1: warning: [-Wforall-identifier]
    The use of ‘forall’ as an identifier
    will become an error in a future GHC release.
    Suggested fix:
      Consider using another name, such as
      ‘forAll’, ‘for_all’, or ‘forall_’.

The warning -Wforall-identifier has been available for three releases (GHC 9.4, 9.6, and the soon-to-be-released 9.8), meaning we can complete the transition in GHC 9.10.

Now forall can become a proper keyword and the warning should become a deprecated no-op.