Add SM as an associated type of ValidStateMachine in pio

[Sympatron]

Since some functions in the pio module expect a ValidStateMachine and some require (PIOExt, StateMachineIndex) it's difficult to write generic code without access to ValidStateMachine::SM. This PR fixes this.

[jannic]

Can you give a quick example where this is useful?
(Not a full example to be added to the repo, just a quick code snipped showing the use case.)

[Sympatron]

Initially I tried to be generic over VSM: ValidStateMachine and use VSM::PIO and VSM::SM to get the the actual PIO and StateMachineIndex instance. But I ran into problems with PioBuilder::build() and changed my implementation to be generic over PIO: PioExt and SM: StateMachineIndex separately. So I don't need it anymore personally, but I still think it might be useful to have and it is not a breaking change, because all implementations of ValidStateMachine are defined in rp2040-hal anyway.

Btw I was working on a software serial implementation using PIO. You can find it here: https://github.com/Sympatron/pio-uart

[jannic]

Perhaps call the type something like SMIdx? This type SM = SM looks a little bit strange, and SM isn't really referring to a specific state machine, only the full tuple (P, SM) does.