Missing impls for Pin<_, DynFunction, _>

[jannic]

There are currently not many useful functions implemented for Pin<_, DynFunction, _>. Therefore, pins configured that way can not be used without converting them to a pin with a statically defined function first. That defeats the idea of having a DynFunction in the first place.

One could, for example, implement OutputPin like this:

    #[derive(Debug)]
    pub enum DynFunctionError {
        InvalidFunction,
    }

    impl embedded_hal::digital::Error for DynFunctionError {
        fn kind(&self) -> ErrorKind {
            ErrorKind::Other
        }
    }

    impl<I, P> ErrorType for Pin<I, DynFunction, P>
    where
        I: PinId,
        P: PullType,
    {
        type Error = DynFunctionError;
    }

    impl<I, P> OutputPin for Pin<I, DynFunction, P>
    where
        I: PinId,
        P: PullType,
    {
        fn set_low(&mut self) -> Result<(), Self::Error> {
            if self.function() != DynFunction::Sio(super::DynSioConfig::Output) {
                return Err(DynFunctionError::InvalidFunction);
            }
            self._set_low();
            Ok(())
        }

        fn set_high(&mut self) -> Result<(), Self::Error> {
            if self.function() != DynFunction::Sio(super::DynSioConfig::Output) {
                return Err(DynFunctionError::InvalidFunction);
            }
            self._set_high();
            Ok(())
        }
    }

It may be useful to first describe use cases for DynFunction where it's not feasible to just use a statically defined function.

[jannic]

(I saved some experimental code to https://github.com/jannic/rp-hal/commits/issue-756/)