Don't pin serialport to a fork. Use upgraded upstream version 4.3.0

[faern]

This PR does not look like much, but it has a bunch of nice outcomes:

• Gets rid of a git dependency. Git dependencies are not good, since the availability of a specific git repo or commit is harder to maintain than crates.io dependencies. Especially since we have not (yet) pushed any tag to the pinned commit even 🙈 (we really should do that in order to be able to build older versions of the testframework)
• Fixes RUSTSEC-2020-0168 by getting rid of the transitive dependency mach that is unmaintained.
• Getting rid of some really outdated and unmaintained macOS bindings.

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This change is 

[dlon]

except the formatting that CI complains about.

Reviewed 4 of 4 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[dlon]

Reviewed 3 of 3 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved