feat: mnemonic profile derive

[kassandraoftroy]

Summary

built on top of and PR into #73

Closes #33 . Overlaps a bit with but does not complete #35 .

Important Design Decisions

• Profiles are what users name. Mnemonics are numbered background seeds (mnemonic 0, mnemonic 1). No edw mnemonic commands, API purposefully references named profiles over mnemonics.

• First unlock of a new instance generates a 12-word seed, creates mnemonic 0 + profile 0, and prints the phrase once. User can rename this profile but it starts with name "default."

• The generate / import / add commands all create exactly one profile at --index (default 0).

• Anti-patterns. Importing a phrase that is already stored errors (use profile add instead). Creating an existing (mnemonic, index) pair errors. Profile display names must be unique.

Storage: mnemonic records (storage index + phrase) and profile records (mnemonic_index, profile_index, optional name) plus a pointer at profile:{m}:{p}. profile_index is BIP44 account m/44'/60'/<profileIndex>'/…. HD keys are not persisted yet.

CLI

profile generate creates a new mnemonic and initializes a profile on the desired --index.

profile import imports a mnemonic and initializes a profile on the desired --index.

profile add adds a new profile index on an already existing/stored mnemonic.

[fbwoolf]

Lgtm!