Unify monadic and pure implementation

The pure implementation should be a specialized version of the monadic code to Identity. Goal: remove code duplication

However, the resulting Core for pure and monadic Identity should be really quite similar to ensure performance doesn't regress in the pure case.

Unfortunately, when I tried to do this last I found the generated code for the monadic code specialized at Identity was wildly different from the pure version.

Notably, `>>=` and friends weren't being inlined. This is critical!

It turns out Identity's Functor, Applicative, Monad instances don't have `INLINE` pragmas (awful!)

I intend to open a CLC proposal to add INLINE pragmas to Identity's instances, pointing to here as a real world example where it was found that the definition wasn't enough.

However, `hegg` will likely still want a solution which doesn't regress performance for older base versions. I'm thinking we could define our own `Identity` type with INLINE pragmas.