ModGC

Ractor: lock around global variable get/set #68

Job	Run time
check (mmtk, release, ubuntu-latest)	9m 1s
check (default, macos-latest)	5m 4s
check (mmtk, release, macos-latest)	6m 54s
check (default, ubuntu-latest)	7m 19s
	28m 18s

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