It's not used outside xmss_commons

Previous code allocated an array on the stack of mlen bytes, but
it should be possible to also sign heap-space messages. By relying
on the fact that sm and m fit the message + signature, we move
the message so that 4*n bytes of prefix can be added.

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!