asyncio: Added async i/o APIs.

examples/asyncio/01-load-bitmaps/README.txt

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+This example code loads a few bitmap files from disk using the asynchronous
+i/o, and then draws it to the window. It uses a task group to watch multiple
+reads and deal with them in whatever order they finish.
+
+Note that for a single tiny file like this, you'd probably not want to bother
+with async i/o in real life, but this is just an example of how to do it.