DEP: Deprecate nonzero(0d) in favor of calling atleast_1d explicitly

* DOC: Mention and try to explain pairwise summation in sum

Note that this behaviour is of course inherited into `np.add.reduce` and
many other reductions such as `mean` or users of this reduction, such
as `cov`. This is ignored here.

Also indicate that the current behavior in this case is deprecated.

This also removes the advice to use `a[nonzero(a)]` or `transpose(nonzero(a))`, for which there are better spellings.

* Added NOTE in numpy\core\fromnumeric.py

DOC: remove performance claim from searchsorted()

* Increment stacklevel for warnings to account for NEP-18 overrides

For NumPy functions that make use of `__array_function__`, the appropriate the
stack level for warnings should generally be increased by 1 to account for
the override function defined in numpy.core.overrides.

Fixes GH-13329

* Update numpy/lib/type_check.py

Co-Authored-By: Sebastian Berg <sebastian@sipsolutions.net>

Fixes #13566

* Unit test for object clip issue

* parametrized test_simple_int32_inout() to
additionally handle scenario where "unsafe"
casting is explicitly passed through, as
requested by reviewer

* add a requested comment related to possibility
of future nan check optimization for `@name@_clip`

* add a unit test to cover the case where np.clip()
is called with None for both max and min

* add a unit test for the case where out is None
and casting is specified as an invalid value, to flush
through code path in TestClip.fastclip

* add unit test + doc update for expected behavior
when amin > amax

* add unit test + patch for bug in npy_ObjectClip; its
operations and operands were wrong; the unit test case
is based on a scenario probed by the hypothesis library

* add unit test for error in timedelta64 MAX
function for clip handling

* add unit test for a pathological case where np.ones(10)
is clipped with amin=1, amax=0

This includes:
* The addition of 3-input PyObject inner loop
* The removal of `->f->fastclip` for builtin types, which now use ufuncs instead
* A deprecation in `PyArray_Clip` for third-party types that still use `->f->fastclip`
* A deprecation of the unusual casting behavior of `clip`
* A deprecation of the broken `nan`-behavior of `clip`, which was previously dependent on dimensionality and byte-order.