Better fix for marking buffers held on the stack #421

Job	Run time
other (ubuntu, jruby-9.3)	43s
other (ubuntu, jruby-9.4)	46s
other (ubuntu, truffleruby)	44s
valgrind	1m 22s
mri (ubuntu, 2.5)	28s
mri (ubuntu, 2.6)	33s
mri (ubuntu, 2.7)	33s
mri (ubuntu, 3.0)	22s
mri (ubuntu, 3.1)	19s
mri (ubuntu, 3.2)	18s
mri (macos, 2.5)	1m 17s
mri (macos, 3.2)	1m 12s
mri (windows, 2.5)	3m 15s
mri (windows, 3.2)	3m 15s
head-versions (ubuntu, ruby-head)	24s
head-versions (ubuntu, jruby-head)	51s
	16m 22s

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