[perl #132042] [CONC] rakudo hangs while concurrently walking trees

[Sam S. via RT]

Even simpler test-case:

    for ^10000 { start say $_ }

For me, this almost always hangs (after printing a few dozen or few hundred 
lines), with the MoarVM version currently used by Rakudo nom:

    MoarVM version 2017.08.1-128-gde6dced

But *no longer* hangs with MoarVM master (which has the fix by ugexe++):

    MoarVM version 2017.08.1-148-g1059eed1

\o/

Marking this ticket as TESTNEEDED.