Image bloat

There are some obvious image bloat issues, I notice that FullAdaptiveStickyImmix won't build due to oversize image.

The stats below show sizes a) for three full heap GCs (differences presumably due allocation paths), b) three copying generational collectors (differences versus full heap presumably due to gen write barrier), c) two sticky collectors (difference due to sticky barrier).

Observations:

1. Immix seems to be bloated (its alloc path is similar to SemiSpace, so should have similar image size).
2. The sticky barrier appears to be particularly expensive, yet it is trivial, so worth investigating.

Data:

Overheads relative to full heap versions are in parens.

SemiSpace: 10.4
MarkSweep: 11.1
Immix: 11.6

GenCopy: 12.8 (2.8)
GenMS: 14.0 (2.9)
GenImmix: 15.3 (3.7)

StickyMS: 14.9 (3.8)
StickyImmix: 15.4 (3.8)