* Implement constant folding rules for Natural (similar to Integer ones)
* Add mkCoreUbxSum helper in GHC.Core.Make
* Remove naturalTo/FromInt
We now only provide `naturalTo/FromWord` as
the semantics is clear (truncate/zero-extend). For Int we have to deal
with negative numbers (throw an exception? convert to Word
beforehand?) so we leave the decision about what to do to the caller.
Moreover, now that we have sized types (Int8#, Int16#, ..., Word8#,
etc.) there is no reason to bless `Int#` more than `Int8#` or `Word8#`
(for example).
* Replaced a few `()` with `(# #)`
