ModGC

briefly defer yielding the thread/gvl #103

Job	Run time
check (mmtk, MarkSweep, release, macos-latest)	2m 7s
check (mmtk, MarkSweep, release, ubuntu-latest)	7m 2s
check (default, macos-latest)	3m 22s
check (default, ubuntu-latest)	5m 21s
ModGC result	4s
	17m 56s

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