There was some slightly overcomplex logic designed to save a couple of hash
operations when R(0) and R(1) were advanced, but the extra code was hard to
understand and didn't save much.

We no longer need to keep track of intermediate values of the counter, which
means we can update it much more easily.

These were left over from when rehash_part did a bunch of logging.

Initialise megolm_cipher via the preprocessor macro, instead of with a
function.