Since there's a tweak being introduced, this should be reflected
in the name of the functions.

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.

The current code results in the same output regardless of endianness.
Verified using a QEMU VM to emulate MIPS.

By prefixing with XMSS_, we hopefully prevent conflicts

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

This also performs numerous consistency fixes

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