As a result, performs various refactors that also impact the verification
function, since cleaner signing functions exposed more overlap.

Using global defines for parameters (as seems to be typical in
academic crypto code) does not play nice with multithreading at all.

A large number of functions was repeated in xmss_fast; these are now
shared between the two implementations via the xmss_commons file.
Notably, we ensure compatability by sharing the verification functions.

Versions are incompatible due to different address formats and differing message compression!