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.

setup.py

@@ -589,6 +589,10 b' extmodules = ['
               ['hgext/fsmonitor/pywatchman/bser.c']),
     ]
 
+sys.path.insert(0, 'contrib/python-zstandard')
+import setup_zstd
+extmodules.append(setup_zstd.get_c_extension('mercurial.zstd'))
+
 try:
     from distutils import cygwinccompiler