ENH: Make signed/unsigned integer comparisons exact

This makes comparisons between signed and unsigned integers exact
by special-casing promotion in comparison to never promote integers
to floats, but rather promote them to uint64 or int64 and use a
specific loop for that purpose.

This is a bit lazy, it doesn't make the scalar paths fast (they never were
though) nor does it try to vectorize the loop.
Thus, for cases that are not int64/uint64 already and require a cast in
either case, it should be a bit slower.  OTOH, it was never really fast
and the int64/uint64 mix is probably faster since it avoids casting.

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Now... the reason I was looking into this was, that I had hoped
it would help with NEP 50/weak scalar typing to allow:

    uint64(1) < -1  # annoying that it fails with NEP 50

but, it doesn't actually, because if I use int64 for the -1 then very
large numbers would be a problem...
I could probably(?) add a *specific* "Python integer" ArrayMethod for comparisons
and that could pick `object` dtype and thus get the original Python object
(the loop could then in practice assume a scalar value).

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In either case, this works, and unless we worry about keeping the behavior
we probably might as well do this.
(Potentially with follow-ups to speed it up.)

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