Coroutine debugging tools

I've been using the coroutine functions, and can't help but notice that debugging coro code is a disaster. Pointers towards doing the following would be good:

- Looking at a coroutine backtrace with the ability to inspect each "stack frame" (inspect local variables, function arguments, etc) (gdb `bt/up/down/frame /info locals` equivalents)
- Looking at all the pending async tasks (gdb `info threads/thread ` equivalent)

The current `co_bt.py` script just shows a backtrace without the ability to inspect any of the frames.