Introduce type MnemonicSize with accompanying tests.

[jonathanknowles]

Issue Number

#357

Overview

This PR:

• adds a MnemonicSize type, which represents the set of valid mnemonic sentence lengths.
• adds ToText and FromText instances, with accompanying roundtrip tests.
• uses this type for the mnemonic generate --size option, instead of String.

This:

• restores the clean separation between parsing and execution.
• allows unit testing of the parsing code.
• produces a more standard error when the parsing fails:

$ cardano-wallet mnemonic generate --size 22
option --size: Invalid mnemonic size. Expected one of: 9, 12, 15, 18, 21, 24.

Usage: cardano-wallet mnemonic generate [--size INT]
  Generate English BIP-0039 compatible mnemonic words.

Comments

[rvl]

I think the way it was before is fine. But this is also fine. MnemonicSize looks a bit like a type level list of Nats ... not sure if we want to go there ...