crypto: various improvements

[ebuchman]

The package has gotten a bit messy and is in need of cleanup. Consider the following.

• clarify what function changed in the bcrypt fork
• fork the correct go-bip39 into here and remove the current bip39 wrapper
• reduce dependencies on tendermint/crypto if/where possible
• mintkey.go should be its own pkg with exposed funcs (doesn't need to be covered by semver tho)
• add support for bip39 passphrases
• add support for generating your own entropy
• keybase needs a cleanup (interface seems bloated)

[ValarDragon]

Why are we forking the bip 39 codebase into the sdk? I'm not opposed to forking but wouldn't this be better as a separate repo so we can get upstream updates

[ebuchman]

Why are we forking the bip 39 codebase into the sdk? I'm not opposed to forking but wouldn't this be better as a separate repo so we can get upstream updates

In this case the repo is small enough that we shouldn't be expecting to pull in anything from upstream.

[jackzampolin]

This has been substantially completed. Going to close this issue. Please reopen individual issues for remaining action items