This is a first step to make WOFF2 a shared library. Public headers are moved
in their own directory to make the public API clearer. This is similar to
the c/include/brotli/ directory in Brotli. "using" commands are also removed
from the public headers, so that callers won't have unexpected side effect
when including the files.
Frédéric Wang
9b5a53b1d7