Use MxcUri instead of String for media URLs.
Fix wrong MXC URIs in tests.
Remove method parse_mxc no longer useful.
Apply new non-exhaustive types: CrossSigningKey, OneTimeKey and SignedKey.
Apply endpoint name change: send_state_event_for_key to send_state_event
When deserialization fails we fallback to providing the user with a
serde_json::RawValue, basically the json string. Ruma should handle all
events that conform to a matrix event shape correctly by either
converting them to their type or returning a custom event.
