Fix active low behavior of GPIO output pins

[MorganR]

When an output pin is in active-low mode, the try_set_low and try_set_high functions currently incorrect behavior.

In keeping with the input behavior for try_is_high and try_is_low, "high" and "low" should refer to the actual output voltage. This works as expected for pins that are not in active-low mode, but if a pin is in active-low mode, then the opposite happens:

• calling try_set_low sets the voltage high
• calling try_set_high sets the voltage low

This bug and the fix have been verified using this test program.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nastevens (or someone else) soon.

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[MorganR]

Done

[bors[bot]]

Build succeeded:

• CI (1.36.0, x86_64-unknown-linux-gnu)
• CI (stable, armv7-unknown-linux-gnueabihf)
• CI (stable, x86_64-unknown-linux-gnu)