ModGC

Use obj flag to differentiate RTypedData and RData #83

Job	Run time
check (default, ubuntu-latest)	7m 47s
check (default, macos-latest)	5m 30s
check (mmtk, release, macos-latest)	10m 6s
check (mmtk, release, ubuntu-latest)	9m 8s
	32m 31s

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