This greatly reduces the memory comsumption of the auth path
computation, since it now also uses treehash. It prevents
duplicate code by re-using the treehash function.

A downside is that it does also pick out the authentication path
during key generation (while it is not used), but this cost is
negligible.

By prefixing with XMSS_, we hopefully prevent conflicts

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