Support for multithreaded tracing

[rikedyp]

It would be good to be able to suspend functions running simultaneously in multiple threads and switch between them using the tracer.

For example:

     ∇ r←Tfn arg
[1]    ⎕←'in thread: ',⎕TID
[2]    ∘∘∘
[3]    {}⎕DL arg
[4]    r←0
     ∇

Then call this with:

Tfn¨2 3

Clicking the tabs in the tracer affects ⎕TID and the visible variables / current stack trace.