rust: speed up zstd decompression by re-using the decompression context
Admittedly, zstd is already pretty fast, but this change makes it
a bit faster yet: it saves ~5% of time it takes to read our large repo.
The actual motivating use case is treemanifest: in treemanifest
we end up reading *lots* of small directories, and many of them
need decompression, and there the saving for [rhg files] is >10%.
(which also seems unreasonable, we should probably keep things uncompressed more)
Mercurial's documentation is kept in reStructuredText format, which is
a simple plain text format that's easy to read and edit:
http://docutils.sourceforge.net/rst.html
It's also convertible to a variety of other formats including standard
UNIX man page format and HTML. You'll need to install Docutils:
http://docutils.sourceforge.net/
Use the Makefile in this directory to generate the man and HTML pages.