Thread creation using async

The async prefix causes a block to be invoked in its own thread:

my $thr = async { #`(do something) };

- - - - - -

• $SLUSH_LEVEL += 1;

• Methods enable the thread to be suspended, rejoined, etc.

• More of a "5.005 threads" than ithreads model
• All variables in scope are shareable
• Locks aren't user accessible.

• This is an area where we need a lot more exploration...
• ...and implementations