pass the target object to command handlers

pass whichever object that has the handler registered in to
the handler function.

When a command handler is registered, check that its arity it correct.
It must accept both a target and an argument string.
implement new command dispatch system

attached existing command handlers to root room

handlers are now dispatched by invoking the command on the user
which propogates to its containing room and so on.

user and room now inherit from serializable-in-room%
which implements both serializable% and in-room-mixin.