Refactor around `wantToUnbox`

I renamed `wantToUnbox` to `wantToUnboxArg` and then introduced
`wantToUnboxResult`, which we call in `mkWWcpr_one` now.

I also deleted `splitArgType_maybe` (the single call site outside of
`wantToUnboxArg` actually cared about the result type of a function, not
an argument) and `splitResultType_maybe` (which is entirely superceded
by `wantToUnboxResult`.