Remove IoPin

[Dirbaio]

As discussed in #340 it's unclear if the current IoPin trait is the best way to go (tldr: due to the fact it changes type it's hard to own it in driver structs).

I propose temporarily removing it, to unblock the 1.0 release. When discussion settles on a final design, we can add it back.

[rust-highfive]

r? @therealprof

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[eldruin]

Thanks! Seems reasonable, though unfortunate, to me in light of #340.
Especially the argument about pins being dropped for good.

TODO if merged:

• Add tracking issue

Further thoughts @rust-embedded/hal?

[eldruin]

Alright, this seems uncontroversial both here and in our weekly meetings. Thank you!
bors r+

[bors]

Build succeeded:

• ci-linux (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux (stable, thumbv6m-none-eabi)
• ci-linux (stable, thumbv7m-none-eabi)
• ci-linux (stable, x86_64-unknown-linux-gnu)
• ci-linux-test (1.54.0, x86_64-unknown-linux-gnu)
• ci-linux-test (stable)
• fmt