Apply two more small fixes to achieve parity with keygen-js
Apply two more small fixes to achieve parity with keygen-js

- argon2u salt
- account for larger seed when deriving from mnemonic

ngzax/urbit,urbit/urbit,belisarius222/arvo,ngzax/urbit,ngzax/urbit,belisarius222/arvo,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,urbit/urbit,joemfb/arvo,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit,joemfb/arvo,ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,ngzax/urbit,jfranklin9000/urbit