Favor optional daisy registers in Pin traits

[mciantyre]

Although daisy registers may be required today, #30 shows that daisy registers may be optional tomorrow. Refactor traits and implementations to always use Option<Daisy> associated constants. This lets us extend chip support without incurring breaking changes.

After #30, it seems that only lpi2c::Pins needs this treatment.

[Florob]

Relatedly, flexpwm::Pin doesn't have any daisy register specified. However, everything except the RT 101x seems to need one.

[mciantyre]

Thanks for noting the missing PWM daisies. That might've been annoying to find when exploring FlexPWM inputs.