Implement BIP-39

[husio]

As developer, I want to implement BIP 39 spec instead of using https://github.com/tyler-smith/go-bip39, so that we do not have to rely on inconsistent and abandoned software with bugs.

This issue is a result of learnings from implementing mnemonic support in bnscli. Used bip-39 package contains bugs and it is not updated for more than a year. If we have time, we should consider either fork & fix or implementing the spec from scratch.

Acceptance criteria

• [ ] provide a package that implements mnemonic and seed generation as described in BIP 39
• [ ] if we work on a fork, send changes upstream to the original repository
• [ ] propose an update to https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki#Reference_Implementation and notify other projects that there is a new BIP-39 implementation in town! :cowboy_hat_face:

[ruseinov]

To be fair, there are some other go implementations already: https://github.com/bgadrian/go-mnemonic/tree/master/bip39 https://github.com/islishude/bip39 https://github.com/landonia/crypto

[kmw101]

Not required as the bnscli is not going to be launched to mainnet