remove claim that other IO modes aren't too hard

Since writing that text, I've made some design decisions that complicate
this:

* dropping a Session uses tokio::runtime::Handle::spawn
* keepalives are tracked with a tokio::time::Sleep
* I support UDP (so multiple sockets per session), which means the
  blocking API would have to use extra threads or async IO behind the
  scenes.

I might make decisions that complicate it further, e.g. having a tokio
task for each socket rather than polling them all in Session::poll_next.

It's of course still *possible* to support async-std and/or a blocking
API if there's demand. It'd require more duplicate logic/APIs now, so
I'm not excited about the possibility.

README.md

@@ -27,8 +27,7 @@ Progress:
 *   [ ] server support
 *   I/O modes
     *   [x] async with tokio
-    *   [ ] async-std. (Most of the crate's code is independent of the async
-        library, so I don't expect this would be too hard to add.)
+    *   [ ] async-std
     *   [ ] synchronous with std only
 *   codec depacketization
     *   [x] video: H.264