ModGC

Autoload encodings on the main ractor #62

Job	Run time
check (mmtk, release, macos-latest)	7m 16s
check (default, macos-latest)	5m 6s
check (default, ubuntu-latest)	7m 31s
check (mmtk, release, ubuntu-latest)	9m 36s
	29m 29s

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