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.
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.