## Issue being fixed or feature implemented
Allow generating 12, 18, 24 words mnemonics. Default to 12 words as it's
the most popular option/de-facto a standard now imo.
## What was done?
Add `-mnemonicbits` option, add tests
## How Has This Been Tested?
run tests, play with wallets on regtest
## Breaking Changes
n/a, old wallets should not be affected
## Checklist:
- [x] I have performed a self-review of my own code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added or updated relevant unit/integration/functional/e2e
tests
- [ ] I have made corresponding changes to the documentation
- [x] I have assigned this pull request to a milestone _(for repository
code-owners and collaborators only)_
