WIP,ENH: Remove np.array fastpaths and move asarray, etc. to C

[seberg]

This simplifies the code (although it adds the complexity of moving
some of the code to C). It also enables METH_FASTCALL when
available on python (which it is in most relevant versions by now)

Note that as of now, the old fastpaths were faster sometimes faster
if METH_FASTCALL is not given. If METH_FASTCALL is used
(available on 3.6+, although not official on 3.6) the new version
should always outperform the old one, by a large margin.

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The first two commits are the basic infrastructure and simple examples and part of gh-15269. The last commit combines all changes related to moving np.asarray and friends to see, removing all special handling.

This requires some changes to the __array_function__ code (with the like= argument) unfortunately. It does also replace some np.array() usages, which I assumed were often micro-optimization since it was faster to use np.array() than np.asanyarray().

Forgot to mark as Draft: This includes changes from gh-15269

[mattip]

Could you modify the top comment for this new version of the PR? It says "The first commit is the new benchmarks, the second is the actual change. The third commit changes ..." Did I miss the benchmarks and the results of running them somewhere?

[seberg]

I posted benchmarks in the initial PR that includes this one. The "benchmarks" this was referring to are the small-array-coercion benchmarks merged more than a year ago :) (this PR is what boosts speeds some up be a factor of 2 – in part underestimating the real factor of probably ~4)

Seems PyPy likes it now 🚀

[seberg]

Arrg, did I end up picking the wrong commit range here? It seems the actual asarray stuff was missing :(. I will open a new PR, sorry about that. (which I guess made this a nice "small" PR though).