DRAFT: anyio port

I wrote a long message but the gh client ate it.

A souped up decouple is used to protect async generators.

After building it I discovered that trio (but not anyio) has something analagous: as_safe_channel.

Some stuff works under trio. I've found stuff that fails though.

I'll need to think more about cancellation; running user callbacks in anyio tasks basically requires the users to use anyio if they want to do cancellation safe cleanup, which is not great. (It is possible though annoying to do cancellation safe cleanup in pure asyncio.)