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.Types or GHC.Prim, which have a weaker guarantee about stability.
We now re-export GHC.Types from GHC.Exts, which should avoid this happening again in the future. In particular, we now re-export Multiplicity and MultMul, which we didn't before. This thus fixes #20695 (closed).
I've taken the opportunity to spruce up the haddocks of GHC.Types and GHC.Exts somewhat, although the way the contents of GHC.Prim are inlined into GHC.Exts is still rather unsatisfactory.