Remove compatibility shims from GHC.Num.Compat (and move them to a better-named module)

[RyanGlScott]

Currently, the GHC.Num.Compat module use quite a lot of CPP to define functions that are intended to work with both pre-9.0 and post-9.0 versions of GHC (9.0 is when the ghc-bignum library was introduced). Nowadays, the minimum tested version of GHC that we use in Cryptol is GHC 9.2, which means that we are no longer building any of the pre-9.0 code paths in GHC.Num.Compat. As such, it's possible that this code has bitrotten.

We should remove all of the pre-9.0 code from this module. After we do this, the name ".Compat" won't be appropriate any more, as there won't be anything compatibility-related in the module. We should figure out a better name for the module at that point (perhaps Cryptol.Utils.Num or something similar).