basic Asyncio support

[jakkdl]

first steps on #208
106, 200, 21x and 9xx are supported, though no autofix.

This is rebased on top of #209, so if reviewing you should only look at the latest commit(s).

todo:

• update readme.md
• update changelog
• do a second pass through the diff, quite tired atm so suspect there may be silly stuff somewhere.
• rewrite descriptions of --anyio / --asyncio to not be dumb. And/or have a --library flag instead.

[Zac-HD]

Looks good so far! Will re-review when you think it's done 👍

[Zac-HD]

let's rebase on master, add ASYNC102 support, and then we're probably done?

[jakkdl]

Uh, I ended up doing A Lot:tm:. But the diff didn't actually end up being all that big. The rebase might break viewing only new changes though, so you might want to review commit-by-commit.

Only ones listed in #208 not supported now is:

• 22X/23X/24X not having tailored error messages (and therefore being untested)
• not detecting asyncio-shielded-timeouts

So I'm now comfortable saying this has decent asyncio support and we could push a release.

TODO: Oh I did rename the repository, so should update links.

[Zac-HD]

LGTM; agree that we could release this and should release whatever we have on the weekend (when I set up release automation again).

I'll let you merge after any docs/links updates you want to make.

[jakkdl]

ah oops, I realized I forgot to ask your opinion on use [trio|anyio].lowlevel.checkpoint() vs use [trio/anyio].lowlevel.checkpoint() (or something else). Both are used in different places in documentation/comments/error messages

[Zac-HD]

I'd probably go with the slash, but don't really care so long as we're consistent.