add maybe-rayon and "threading" feature

[FreezyLemon]

Supersedes #198 (see also image-rs/image#2028). I just copied the way rav1e does it.

It is very important that downstream users that use default-features = false manually enable the "threading" feature to keep using real rayon.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 21.98%. Comparing base (d39df15) to head (a826919).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #206   +/-   ##
=======================================
  Coverage   21.98%   21.98%           
=======================================
  Files          36       36           
  Lines        3553     3553           
=======================================
  Hits          781      781           
  Misses       2772     2772           

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[shssoichiro]

Seems fine, just will need to remember this is a breaking semver change.