Undo resolver changes and instead fall back to "old logic"

If only output dtypes are provided, we add the additional step
in the ufunc type resolver of converting a signature of the format::

    (None,) * nin + (DType,) * nout

into::

    (DType,) * (nin + nout)

And try searching for a matching loop again.  The advantage of this
is that IF the ufunc has mixed-type inputs, we have a fighting
chance of finding a good loop for `dtype=DType`.
Further, future `promoters` could refine such logic, while at the
same time, we do not need to use completely different code paths
for `signature=...` and `dtype=...`.

The rare downside is that we might pick a `dd->d` loop when a
`dl->d` loop would do (SciPy has such ufuncs).
The up-side is, that before, using `dtype=...` was not even possible
for functions with non-homogenous loops like `np.ldexp`.

The future will hopefully see more precise promotion logic, and some
of the more surprising paths simply deprecated...

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