This allows different backends to store additional state information
in the secret key while the rest of the codebase remains agnostic.

In particular, this prepares for a common xmss_core.h API for both
the standard and the BDS-traversal-based implementations.

This produced repeated indices when reaching 2^32 signatures.
This was introduced in 9b35b00d98
with the re-introduction of runtime parameters. Compile-time parameters
did not contain this error.

This makes it easier to mix and match with other implementations
for compatibility testing.

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