Include some last feedback from Ralf and Ben


Co-authored-by: Ralf Gommers <ralf.gommers@gmail.com>

This makes sure the signature is complete everywhere and for now
renames `userdata` to `innerloop_data`, which may be less confusing
with regards to the previous (but different) userdata.

Rather than saying 'NEP 4x', the links
in the note at the top now include the NEP titles as well.

* NEP: Initial draft for NEP 43

* NEP: Small updates to NEP 43

* Partial iteration on some content, mostly updates the calling convention

Its not quite settled, I don't quite like the `npy_intp *` as a default
"user-data" yet, but I think I may warm up to it, and I think some kind
of default like that is really very convenient.

Split out `user_data *`, since I think Marten has a point that
separating "constant" and volatile data is conceptionally easier.

* Add a bit more overview-intro at the start and refactor "bound" method section

* Small fixups based on Marten and also Hameers comments

* Revise the "promotion and dispatching" section a bit.

* Fix back-compat section and add a tiny bit of implementation

* WIP: Further updates nep 43

* Revise NEP 43 and expand on arraymethod (previously the dispatching/promotion was more fleshed out)

* Update image and make sure it is included at the right place.

* small fixups

* Update doc/neps/nep-0043-extensible-ufuncs.rst

Co-authored-by: Dave Hirschfeld <dave.hirschfeld@gmail.com>

* Fix comment which was missing the final part of sentence

* Pass through, up to "steps involved in a ufunc call"

* Review up to ArrayMethod

* Review up to ArrayMethod Specs

* Review up to loop reuse

* Review most of the remaining document

* Fix title typo

* Wording updates and minor fixups.

* Apply suggestions from code review

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Address Ross's additional comments

Co-authored-by: Dave Hirschfeld <dave.hirschfeld@gmail.com>
Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* DOC: Revise NEP 42 "common DType" section

* DOC: Eliminate duplicate doc in NEP42

* DOC: Fix missing 'not' in NEP42 edit

* DOC: Minor typos in NEP42 edit

* DOC: Reword opening sentence of NEP 42 revision

Also clarify a wording.

* DOC: Correct :class: reference in NEP42 edit

* DOC: Fix Windows line-enders on NEP42 edit

* DOC: New round of common DType edits of NEP 42

* DOC: Uniquify anchors in NEP 42

* DOC: Typo in NEP42 edit

* DOC: Last push for NEP 42 edit

* Small fixups and moving example

One paragraph on `__common_dtype` in the alternatives was already
before very confusing. This makes it hopefully more clear.

* NEP: Change title to "New and extensible DTypes"

Co-authored-by: Sebastian Berg <sebastian@sipsolutions.net>

* NEP: update NEP 42 with discussion of type hinting applications

As discussed in https://github.com/numpy/numpy/pull/16759, the new
DType classes provide a good path forward for making `ndarray` generic
over DType. Update NEP 42 to discuss those applications in more
detail.

* NEP: discuss typing for use scalar types in NEP 42

Also clean up the language a bit.

to provides -> to provide