Overhaul the serialization system to create one mmapable file per directory
and distribute bytes for each directory from a single malloc for that directory. Store pointers as differences between the data pointed to and the pointer's address (s_off = s - v). Don't serialize data structures that never actually get serialized. Separate strings used for keys from strings used for values (in FcPatternElt and FcValue, respectively). Bump FC_CACHE_VERSION to 2.