It's now used for both XTEA and ISAAC keys, and there's nothing
XTEA-specific about it so there's no need to duplicate it.
Signed-off-by: Graham <gpe@openrs2.org>
It supports reading and writing a cache backed by a Store, as well as
standalone .js5 files.
I'm not quite as happy with this as I am with the lower-level API yet,
and there are still a few remaining TODOs.
Signed-off-by: Graham <gpe@openrs2.org>