Support multiple wallets

As a user, I want to be able to switch between different wallets.

- Everyone starts with a `default` wallet, and all current commands still work the same;
- Using `sw wallet`, a user is able to manage their wallets. Some sub-commands:
- `sw wallet create NAME`: create a blank wallet with name NAME
- `sw wallet rename OLD NEW`: rename wallet OLD to NEW
- `sw wallet use NAME`: switch to NAME
- `sw wallet merge SRC DST`: move SRC's contents to DST and destroy SRC
- `sw wallet destroy NAME`: permanently delete wallet NAME and its contents (needs confirmation)
- Storage: save each wallet as `NAME.wallet` and load the wallet specified in `current_wallet`, or `default`