Replace all of the bank/id pairs with simple offsets, recode several
data structures to always use offsets inside the library to avoid
conditional paths. Exposed data structures use pointers to hold offsets,
setting the low bit to distinguish between offset and pointer.
Use offset-based data structures for lang charset encodings; eliminates
separate data structure format for that file.
Much testing will be needed; offsets are likely not detected everywhere in
the library yet.
Map existing architecture signature to short architecture name
at build time. This architecture name is (as yet) unused, but will be used
to build per-architecture cache files with names made unique by including
the architecture name. The auto-detected architecture name can be overridden
with the --with-arch=ARCH configure option.