Support undo

If OT::apply were to have a sibling method OT::apply_reversable(...) which returned a tuple (Doc, reverse_op), where reverse_op applied to Doc would result in the inverse method, then undo could be possible.

edit-text/edit-client/src/state.rs

Lines 274 to 275 in 55003ee

    
           // TODO Generate an "undo" version of the operation and store it. 
        
           // This should come from the Op::apply above.

Here the undo would be generated, and then attached to the self object. Each time an operation is performed, this undo is replaced (or queued, for multiple-level undo) or if we have incoming packets, the undo packet is transformed against the incoming packet.

	// TODO Generate an "undo" version of the operation and store it.
	// This should come from the Op::apply above.