Support use of abbreviated 4-letter BIP39 words

The BIP39 wordlists are selected so that the first 4 letters of each word are unique. The functions for working with mnemonics should accept them in that form, and expand them to the full words.

This is important because tools like the CryptoSteel / BillFodl metal key storage devices only accept the first four letters of each word, and making the user hand-expand that back to the whole word is painful and unnecessary.

Screwing around with the input in this way playing guess-what-I-mean is fine because of the checksum. As long as the checksum passes, we know we performed the correct expansion. (In light of this I think we should also offer user-friendly things like automatic casefolding to the same case as the dictionary. All of this should only be triggered after trying the entered string as-is and failing the checksum in that form.)