IrcMsgPrefix into_owned -> to_owned

Follows conventions
Start rewriting parser for arbitrary encoding and safety

Message bodies will allowed to be encoded with arbitrary
encodings.

We will gain message type safety, so ping will have at least
one argument, PRIVMSG will have at least a target and a message,
etc.
Allow deprecated APIs temporarily.

Once we land the new parsing code, it most of the usage
of deprecated code should be eliminated.