Concurrency autodetection

[lovelydinosaur]

Adds autodetection of aync library via the neat little sniffio package.
Also adds support for a simpler backend=<str> style.

Valid usages include:

client = Client(backend=TrioBackend())
client = Client(backend="trio")  # Or "asyncio" or "auto".
client = Client()  # Equivelent to `backend="auto"`