Revert bcrypto.GenerateFromPassword() to mainline go x/crypto

[alessio]

As per tendermint#3027.
This change is required for go dep -> go mod migration.

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• Wrote tests
• Updated relevant documentation (docs/)
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[ebuchman]

I don't think this will work - see #3129.

Does modules not support aliasing the repo? To properly fix this we'll need to move a real encryption scheme rather than abusing bcrypt like we are now. Bcrypt is for password hashing, not deriving secret keys.