Allow the end user to add new constructors

[marcosh]

Currently the StateMachine type has a predefined set of constructors which provide some basic combinators to build more complex machines.

I'm definitely not sure that this set is complete in any way, so it could be interesting to allow end users to add new ways of creating a state machine.

Some possible ideas which could be interesting to experiment:

• define a StateMachine with its running function, similarly to Mealy
• use a technique to make data types extensible, like trees that grow or data types a la carte (I don't know this stuff, so it might not be we is needed)