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.

By prefixing with XMSS_, we hopefully prevent conflicts

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

Previously this was generated and thus gitignored