NUT-XX: Deterministic NUT-13 Derivation Paths

[vnprc]

Ehash will use a different currency unit for each mining epoch for each pool. We need a way to dynamically and predictably map currency unit names to derivation paths.

[vnprc]

comments from nutcore meeting:

• add section to deterministic secrets spec in place of a new nut
• add 10,000 reserved units range
• confusing: mint or wallet spec? (this impacts the above 2 points)
• if it's for wallets then reserved numbers are important, for mints not important
• I think this advice was for @lescuer97 (idk?): start spec from BIP32 seed phrase
• how to handle collisions?