`core.Keyring`'s API could be better

[mrnerdhair]

core.Keyring's API has some room for improvement:

• The API doesn't align with the interface a developers might expect for a standard collection
• The API contains some unused functionality (like .exec())
• The event emitting functionality isn't well-encapsulated or documented
  • Some wallet implementations mutate the keyring and then emit events like connect/disconnect; some don't
  • From a type perspective, the keyring should in principle be able to tell you what types of events it will emit or its API isn't well-defined

AC:

• core.Keyring's API should mirror the standard Set interface
• core.Keyring should emit its own connect/disconnect events, consistently, when adding/removing a wallet

Fixed by #436.