ModGC

Use obj flag to differentiate RTypedData and RData #82

Job	Run time
check (mmtk, release, ubuntu-latest)	5m 2s
check (default, ubuntu-latest)	4m 54s
check (default, macos-latest)	5m 44s
check (mmtk, release, macos-latest)	4m 42s
	20m 22s

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