Some way to dump debug info about all living tasks

[lilyball]

There should be some way to print debug info about all tasks that are still alive. The use-case I have for this is figuring out why my app didn't exit (because at least one task is still alive). This could be the default behavior for e.g. the USR1 signal, or it could be provided as a debug method that I can then call from my own signal handler.

[lilyball]

cc @alexcrichton

[thestinger]

With native tasks, it's easy to inspect them by attaching a debugger or using OS-specific mechanisms like /proc on Linux.

[thestinger]

I think attaching a debugger is a satisfactory solution now that #17325 is filed / approved. In gdb, the command for this is info threads. #10302 would be easy to implement and would provide the task names there.