Several significant improvements to the MMTk debugging Harness.
Now maintains a 'shadow stack' in the heap, to support collectors that do
deferred tracing of roots.
The deterministic scheduler yield algorithm no longer changes its behaviour
when watchpoints, tracing etc are enabled.
Watchpoints can now be enabled on variables in the test scripts
Scripts can specify harness options, especially heap size. Each plan has
a configured 'heap factor' which is used to scale the specified heap size
to allow for copy reserves etc.
Short names for plans are now implemented in the harness rather than in the
wrapper script. This simplifies command lines.
New intrinsic to return the number of GCs that have occurred. The SpreadAlloc
scripts have been updated to perform a certain number of GCs.