[ecash-lib] Add mnemonic functionality

Summary:
Adds entropyToMnemonic, mnemonicToEntropy and mnemonicToSeed, which is important to build a wallet.

The wordlist is included in the bundle but has to be imported manually to give users maximum flexibilty without forcing everyone to blow up their bundle size.

Depends on D17610.

Test Plan: npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D17612