Switch `inj _` to `inj f`, part 2

Code not affected by a00d9bd8.

All occurrences are gone, except for one in `base.v` where you'd need different
functions.

However, I'm unsure this is an improvement: in lots of cases here, the function
didn't need to be guessed, but could be deduced by "simple" higher-order
unification, the one where unifying `?f ?a` against `g args last_arg` sets `?f =
g args`.