``clip`` now uses a ufunc under the hood

This means that registering clip functions for custom dtypes in C via

`descr->f->fastclip` is deprecated - they should use the ufunc registration

mechanism instead, attaching to the ``np.core.umath.clip`` ufunc.

It also means that ``clip`` accepts ``where`` and ``casting`` arguments,

and can be override with ``__array_ufunc__``.

A consequence of this change is that some behaviors of the old ``clip`` have

been deprecated:

* Passing ``nan`` to mean "do not clip" as one or both bounds. This didn't work

in all cases anyway, and can be better handled by passing infinities of the

appropriate sign.

* Using "unsafe" casting by default when an ``out`` argument is passed. Using

``casting="unsafe"`` explicitly will silence this warning.

Additionally, there are some corner cases with behavior changes:

* Padding ``max < min`` has changed to be more consistent across dtypes, but

should not be relied upon.

* Scalar ``min`` and ``max`` take part in promotion rules like they do in all

other ufuncs.