Remove global rmem page slab #516

Job	Run time
other (ubuntu, truffleruby)	47s
valgrind	6h 0m 15s
head-versions (ubuntu, ruby-head)	32s
other (ubuntu, jruby-9.4)	43s
head-versions (ubuntu, jruby-head)	1m 1s
mri (macos-latest, 2.6)	43s
mri (ubuntu-latest, 3.0)	42s
mri (ubuntu-latest, 3.3)	40s
mri (macos-latest, 3.2)	27s
mri (ubuntu-latest, 3.1)	6h 0m 14s
mri (ubuntu-latest, 2.6)	33s
mri (ubuntu-latest, 2.7)	47s
mri (ubuntu-latest, 2.5)	35s
mri (ubuntu-latest, 3.4)	46s
mri (ubuntu-latest, 3.5)	42s
mri (ubuntu-latest, 4.0)	30s
mri (windows-latest, 3.2)	2m 18s
mri (ubuntu-latest, 3.2)	38s
mri (windows-latest, 2.5)	2m 13s
	12h 15m 6s

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