gc_massacre branch is ready for testing.

[Nick Wellnhofer]

First of all: Great work! This cleans up the whole GC code considerably 
and makes it much easier to understand and work on.

I just tested the branch on Ubuntu 10.04, 32bit, Athlon 64 X2, 2.3 GHz. 
It builds fine and all tests pass. A Rakudo build completes in 3m59s and 
has a maximum RSS of 623MB. The numbers for trunk with different 
--gc-threshold parameters are:

Threshold  25%: 310MB, 4m55s (default)
Threshold  50%: 413MB, 4m25s
Thershold  75%: 494MB, 4m15s
Threshold 100%: 578MB, 4m11s

AFAICS, these results can be explained by the provisional static 
threshold of 256MB in gc_ms2. I think this should be worked out before 
merging. Other than that, I don't see any problems.

Nick