Re-export GHC.Types from GHC.Exts
Several times in the past, it has happened that things from
GHC.Types were not re-exported from
GHC.Exts, forcing users to import either
GHC.Prim, which have a weaker guarantee about stability.
We now re-export
GHC.Exts, which should avoid this happening again in the future. In particular, we now re-export
MultMul, which we didn't before. This thus fixes #20695.
I've taken the opportunity to spruce up the haddocks of
GHC.Exts somewhat, although the way the contents of
GHC.Prim are inlined into
GHC.Exts is still rather unsatisfactory.