Re: list-threads

[Gemini Lasswell]

I've just pushed an update to the scratch/list-threads branch which
includes:

- lisp/thread.el, the new home for thread-related Lisp functions.

- A 'b' command in the *Threads* buffer which shows a backtrace of where
  the thread at point had yielded or was blocked at the moment you
  pressed 'b'.

- Documentation and a NEWS entry.

- Tests.

There are no local variables in the thread backtraces.  I experimented a
bit with making Fbacktrace__locals work on non-current threads, with
mixed results (success in extracting variable values at the cost of
corrupting something making Emacs prone to crashing afterwards), and
decided that feature is more difficult (for me, at the present time)
than it is important.

If you have any feedback, let me know, otherwise I'll merge this in a
few days.