BIP39 compliance clarifications

This pull request makes purely cosmetic changes, but has the primary intention of making this modules compliance to BIP39 more clear, as the code was slightly difficult to decipher on first glance. 

Less `Buffer`s being created so its probably faster too; but eh.

I know you'll love dc4e1ff @weilu, but 6488b63, 1f6915b and b41eefd are the real meat of this PR.