ModGC

wip - remove broken assertion for now, see what else breaks #339

Job	Run time
check (default, macos-latest)	5m 23s
check (default, ubuntu-latest)	6m 51s
check (mmtk, release, macos-latest)	6h 0m 19s
check (mmtk, release, ubuntu-latest)	4m 52s
	6h 17m 25s

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