Re: Popcount optimization using AVX512

Here is a v14 of the patch that I think is beginning to approach something
committable.  Besides general review and testing, there are two things that
I'd like to bring up:

* The latest patch set from Paul Amonson appeared to support MSVC in the
  meson build, but not the autoconf one.  I don't have much expertise here,
  so the v14 patch doesn't have any autoconf/meson support for MSVC, which
  I thought might be okay for now.  IIUC we assume that 64-bit/MSVC builds
  can always compile the x86_64 popcount code, but I don't know whether
  that's safe for AVX512.

* I think we need to verify there isn't a huge performance regression for
  smaller arrays.  IIUC those will still require an AVX512 instruction or
  two as well as a function call, which might add some noticeable overhead.

-- 
Nathan Bossart
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