TYP: get_reverse_indexer, get_group_index_sorter
nailing down all the "np.intp"s bit by little bit.

AFAICT we're not going to be able to annotate ndarrays with ndim/dtype any time soon.  Might be worth looking into https://pypi.org/project/nptyping/
any perf changes? > any perf changes?

might be some small gain on 32 bit, nothing on 64. gentle ping; the intp_t stuff (this, #40475, #40474) is becoming blocker-ish for getting algos.pyi and groupby.pyi  yeah will look a bit later