Full HMM support for circuit padding state transition

The circuit padding state transition system is quasi-deterministic. A non-deterministic state machine can be built from this using many states through clever use of the infinity bin, but this will lead to excessively complicated machines.

One can imagine a non-deterministic state machine/HMM that simulates an entire web browsing session on a circuit. If this kind of machine seems useful, it may be better simply to transform the circpad_state_t.next_state transition field into an array of structs that specifies the next state as well as a probability for transitioning to that state for that event.

We would accept patches for this modification if it seems useful.