Support `try_into` for converting from enum to specific type

enum_dispatch generates implementations of From<SpecificType> for GeneralType that are used in the documentation like so:

let (a_linear_knob, a_logarithmic_knob) = some_existing_knobs();

let knob = Knob::from(a_linear_knob);
let knob = Knob::from(a_logarithmic_knob);

However the inverse operation is not generated, that is TryFrom<GeneralType> for SpecificType

which could be used like so:

let knob = Knob::from(a_linear_knob);
let linear_knob: LinearKnob = knob.try_from().unwrap();