Make catch#, keepAlive#... only levity polymorphic

This MR changes the types of continuation-style primops to enforce that the return representation is boxed.

Affected primops:

• catch#
• keepAlive#
• maskAsyncExceptions#, unmaskAsyncExceptions#, maskUninterruptible#

For example, the type of keepAlive# is changed from

  forall
    {a_lev} {b_rep}
    (a :: TYPE (BoxedRep a_lev)) (b :: TYPE b_rep).
      a -> State# s -> (State# s -> b) -> b

to

  forall
    {a_lev} {b_lev}
    (a :: TYPE (BoxedRep a_lev)) (b :: TYPE (BoxedRep b_lev)).
      a -> State# s -> (State# s -> (# State# s, b #)) -> (# State# s, b #)

Technically, we could allow a slightly more general type by insisting that the representation of b fit into a single machine register, but enforcing this restriction would be a significant amount more work when it isn't clear that anyone would benefit from it.

TODO:

• Check that this doesn't impact user programs.
• Updates notes which mention the type of these primops.
• Update changelog.
• Write a CLC proposal.