ModGC

briefly defer yielding the thread/gvl #104

Job	Run time
check (mmtk, MarkSweep, release, ubuntu-latest)	10m 35s
check (mmtk, MarkSweep, release, macos-latest)	12m 10s
check (default, macos-latest)	9m 29s
check (default, ubuntu-latest)	8m 19s
ModGC result	3s
	40m 36s

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