The reference implemention of XMSS currently assumes that n bytes of padding is used for the prefix in the functions prf, hash_message, thash_h, and thash_f. While this is the case for all of the parameter sets in RFC 8391, the draft version of NIST Special Publication 800-208 specifies paramter sets in which the amount of padding is different than n.
This commit allows for the padding length for a parameter set to be specified separately from n.
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.
