idea: use sys.set_coroutine_wrapper in the test suite to enforce assert_yields *everywhere*

[njsmith]

Like:

@types.coroutine
def enforce_checkpoint(coro):
    with assert_yields():
        yield from coro.__await__()
sys.set_coroutine_wrapper(enforce_checkpoint)

and calling this from conftest.py

[njsmith]

There are some async functions in trio (esp. in the test suite) that intentionally don't have any checkpoints. The way to handle this I think is to annotate those with @no_checkpoint or something (which just sets a flag that the enforce_checkpoint function would look for), and this would actually be a good piece of executable documentation, as well as being essentially free at runtime.