Bug 564016 - Include c:prefix in typelib, use it to optimize find_by_gtype

Bug 564016 - Include c:prefix in typelib, use it to optimize find_by_gtype

Parse the c:prefix from the .gir, include it in the header.  Armed with this
information, we can now optimize lookups of GTypes because we
have the requirement that GTypes must start with the c:prefix.  We do
fall back though if a lookup fails.