Overhaul the serialization system to create one mmapable file per directory

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.