GPIO: Don't rely on embedded-hal-traits

[David-OConnor]

Ie, remove unwrap() requirement when setting pins in both internal and external code. Note how other peripheral modules like SPIM and TWIM add the embedded-hal impls as add-ons, vice primary API.

I'll submit a PR if you agree.

[jonas-schievink]

Yeah, we can probably add inherent methods for controlling the pins. Note that there's also a pending redesign of the whole GPIO module in #301.