Dropped Curio support

[agronholm]

Curio has become a liability due to its policy of only allowing coroutine functions to interact with the curio kernel.
Dropping support will allow AnyIO to better match the Trio APIs and to allow more functionality for apps that still rely on synchronous callbacks due to existing asyncio libraries.

See #179 for discussion.

[belm0]

Perhaps it's worth trying to say something objective about Curio in the readme or docs.

Why not Curio?

Curio is another async framework which early versions of anyio tried to support along side of asyncio and Trio. The Curio design is quite strict about every API call being async (Event.set(), get_current_task(), etc.), which hinders bridging to synchronous code and increases the general complexity of concurrent programs. Lacking strong demand for Curio support, anyio elected to forgo it and allow the advantages of the partly-synchronous aspects of asyncio and Trio to be available in the API.

[thedrow]

I completely agree with this decision.