ModGC

Fixes to encoding/transcoding for ractors. #1444

Job	Run time
check (default, macos-latest)	5m 36s
check (mmtk, release, macos-latest)	8m 7s
check (default, ubuntu-latest)	7m 10s
check (mmtk, release, ubuntu-latest)	9m 21s
	30m 14s

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