This ensures that xmss_core and xmss_core_fast offer the same API.
Note that xmss_core_fast still needs a major refactor, and this
wrapper is not exactly very clean. There is a considerable chance
this refactor will change the format of the state in the secret key.

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