ModGC

support nested VM barrier synchronization #43

Job	Run time
check (default, ubuntu-latest)	44m 21s
check (mmtk, release, ubuntu-latest)	5m 5s
check (default, macos-latest)	44m 24s
check (mmtk, release, macos-latest)	3m 52s
	1h 37m 42s

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