Simply, it returns the number of faces on a font blob.

To be used on hb-sanitizer tool but other clients also
can benefit from it.

New API:
hb_face_get_table_tags()

Fixes https://github.com/behdad/harfbuzz/issues/560

This prepares the headers for exporting symbols using visibility
attributes or __declspec(dllexport), so that we do not need to maintain
symbols listing files, as this is what was and is done in GLib and GTK+.

Makes documentation / introspection easier.