util: expose an "available" API on compression engines When the zstd compression engine is introduced, it won't work in all installations, namely pure Python installs. So, we need a mechanism to declare whether a compression engine is available. We don't want to conditionally register the compression engine because it is sometimes useful to know when a compression engine name or encountered data is valid but just not available versus unknown.

setup: compile zstd C extension Now that zstd and python-zstandard are vendored, we can start compiling them as part of the install. python-zstandard provides a self-contained Python function that returns a distutils.extension.Extension, so it is really easy to add zstd to our setup.py without having to worry about defining source files, include paths, etc. The function even allows specifying the module name the extension should be compiled as. This conveniently allows us to compile the module into the "mercurial" package so "our" version won't collide with a version installed under the canonical "zstd" module name.