Speed up (~3 times) GCM by switching to unsigned long instead of char arrays

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Big improvements on GCM. Implemented fast mode, costing 256 bytes/ctx.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>