Call for reviewers and testers - accelerated file copy API

There are some PRs open that would benefit from volunteers reviewing
and/or testing them.

https://urldefense.com/v3/__https://github.com/bcpierce00/unison/pull/577__;!!IBzWLUs!TbQJjIb5eRiD8ITAyW9NfEBA0uuSroLR_XbJnQ1_eQbmmMR5UtLfck8mRMeQYRaLnTrwr4GjS7MzRUOxgbc5w2aEomY$ 

This PR is about making use of OS-specific syscalls to accelerate file
copy operations, ideally skipping the copy entirely and doing a clone
operation (reflink) either for the entire file or on the block level.

Notice that while only local file copy operations can be accelerated,
also a remote sync makes use of local copies in some cases (to skip a
network transfer if a file with the same contents already exists in
the other replica, for example).

https://urldefense.com/v3/__https://github.com/bcpierce00/unison/pull/876__;!!IBzWLUs!TbQJjIb5eRiD8ITAyW9NfEBA0uuSroLR_XbJnQ1_eQbmmMR5UtLfck8mRMeQYRaLnTrwr4GjS7MzRUOxgbc5-HW4ScQ$ 

This PR enhances the rsync protocol by using the new accelerated fily
copy functions added by the PR above. If you choose to review this PR
then notice that the first commit is the same between the two PRs.

If you'd like to test this new code but are unable to build yourself
then you can download CI-built binaries as follows:
- on the PR page, select the Checks tab (just above the PR description)
- once on the Checks page, you should see a dropdown called Artifacts
  (just above build logs); use that to find a suitable binary (hover
  over filenames to see which one works for you)
- depending on the page you land (if you see a build graph instead of
  a build log), the downloadable artifacts may also be listed at the
  very end of the page