Switch to BIP39 word list by default

[afontenot]

Improves security slightly with default settings (16 bits -> 22 bits), and despite this the average word length is shorter (7.65 -> 5.40 chars) and (IMO) more easily remembered, communicated, and spelled.

[afontenot]

I'm marking this as a draft because it's definitely unfinished (needs work on tests, etc), and also some feedback.

I realize BIP39 may not be the best theoretical wordlist approach, or anything like that, but IMO it's an easy change that's superior in every single way over the existing word list. It's worth moving to, even if it's ultimately temporary, for an easy win.

If you're interested in merging this, what would you like to do with the existing word list? Should it be left as dead code for some future word list switching feature? Removed?

[piegamesde]

I'd like to point out https://github.com/magic-wormhole/magic-wormhole.rs/issues/106 and https://github.com/magic-wormhole/magic-wormhole-protocols/issues/31

[afontenot]

Right, I had seen #106 before. I certainly see the appeal of three or four letter codes. My goal here was simply, while further changes are brewing, to go ahead and get an easy win while maintaining the current number-word-word format that users are used to.