UART wrapper (chrysn's cleanup)

[chrysn]

This PR just contains fixes relative to #39; let's keep discussion there, but it's a PR so that we can see what CI says.

[chrysn]

I've made a bit of a mess with which branch to push to, and GitHub's view of when things were pushed is inconsistent -- it shows "Use From for error conversion" before your review, but actually I don't think you've seen them. I don't think they are major, but maybe you'd like to revisit (best viewed individually)

• a481892
• f749c35
• 4432771 + 9c3186d
• c819fdc

and the recent 9e052af (but that's just sequence changes)

I'll do some squashing, trying to strike a balance between documenting the genesis and keeping small fixes out of the way; I'll force-push the tree-identical result.

[chrysn]

Squashed now into what I think is a sensible sequence of ocmmits, leaning towards keeping history.

• GitHub is said to be a bit annoying when @-mentioning users in commit messages; removed the @.
• Applied cargo fmt to all commits using git filter-branch -f --tree-filter 'cargo fmt; git add **/*.rs' fa66f7f0eec46679bf6c031f1d2588068c323972.. (This has no impact )
• Squashed fixes where they are simple oversights
• Squashed groups of commits where there was no significant difference. Note that for applying reviewers' comments, there is a batch feature in some (but not all) GitHub views of a PR
• Reordered commits to reduce the chances of build failures (even though I don't expect to ever have a bisect hit in here, given that as a whole the PR just introduces a feature)

[chrysn]

As a last-minute removal I had to ditch the get_[rt]x_pin methods -- those are just not available consistently in RIOT across boards; it's a niche feature probably only tested wherever anyone had a concrete use case and implemented it on the boards, and this would be the first time it is being checked for across the boards.

Enabling auto-merge already; that will fail anyway if there is any follow-up trouble (which I don't expect).

[kbarning]

Sorry for not answering, I was very busy during the last weeks. Is there anything left to do @chrysn?

[chrysn]

Sure, that was a time period where one is bound to get mixed availabilities.

I think all is done here :-)