This also reduces some duplication between XMSS and XMSSMT

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