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.
