This enables building with NumPy on Linux and macOS. Windows support
should be complete to, but is untested as of now and may need a few
tweaks. This contains:
- A set of `meson.build` files and related code generation script
  tweaks, header templates, etc.
- One CI job on Linux
- Basic docs on using Meson to build NumPy (not yet integrated in the
  html docs, it's too early for that - this is for early adopters right
  now).

The build should be complete, with the major exception of SIMD support.
The full test suite passes. See gh-22546 for the tracking issue with
detailed notes on the plan for switching NumPy to Meson as its build
system.

Co-authored-by: Stefan van der Walt <stefanv@berkeley.edu>

Also remove the unused `cythoner.py` script. Meson itself has sufficient
Cython support now for what we need. And the `-3 --fast-fail` flags are
the default already.

Remove support for Apple Accelerate, since it is buggy. A build error should
occur on most or all setups if linked against Accelerate. Test or import failures
should occur on setups where Accelerate is picked up dynamically.

Co-authored-by: Warren Weckesser <warren.weckesser@gmail.com>

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

BUG: Use C call to sysctlbyname for AVX detection on MacOS