[Stackless] How to "transfer" a tasklet to the current thread

[Kristján Valur Jónsson]

Okay, I have pushed new changes.
bind_thread() now requires the target to be non-runnable.
In the process, I fixed a number of issues:
tasklet.remove() didn't work correctly for a tasklet from a different thread.
new tasklets (that hadn't run yet) were not killed when their thread went away.

I added more unittests.  Give it a spin.
K