zstandard: vendor python-zstandard 0.12 The upstream source distribution from PyPI was extracted. Unwanted files were removed. The clang-format ignore list was updated to reflect the new source of files. test-repo-compengines.t was updated to reflect a change in behavior of the zstd library. The project contains a vendored copy of zstandard 1.4.3. The old version was 1.3.8. This should result in some minor performance wins. # no-check-commit because 3rd party code has different style guidelines Differential Revision: https://phab.mercurial-scm.org/D6858