Pass pointer to processor instance to handlers

So that handlers have the ability to respond to the messages they
receive.

The proccessor pointer is the first implicit argument to the handler.
Add support for "muted" message processor handlers

They're called "muted" because they don't take a pointer to the
processor instance as the first argument which means they're unable
to respond to the messages they handle.

This is mostly useful for saving keystrokes when writing handlers.