Replace elixir bip39 implementation

Follow up to #883

Advantages:

• 100% test coverage + CI set up
• smaller tests that strictly address properties of the specs
• documentation
• vectors.json identical to Trezor's + tests for seed verification with a passphrase
• idiomatic code, no compilation warnings, no unnecessary conversions to strings containing binary numbers
• flexible interface, e.g. works with raw binaries as well as hex-encoded
• does not output 1.5M of text when running tests
• does not confuse entropy with a private key
• no commented out code
• descriptive commit log

@prusnak let me know if you'd like me to squash the commits