Unlifted bools

[daig]

We'd like to provide instantiations for type Bool = Int# and type Bool = (# (# #) | (# #) #) but a number of limitations with primitive haskell and backpack make this impractical.

In particular:

• Allow levity polymorphism in binding position blocks a useful implementation of bool :: (a :: TYPE r) -> a -> Bool -> a

• Abstract pattern synonyms (for hsig and hs-boot) blocks us from exporting any constructors, since we can't instantiate true, false :: Bool at an unlifted type, but we can implement pattern synonyms pattern True :: Bool, pattern False :: Bool, if only we could declare them in signatures.