Add splay GC benchmark ported from V8/WebKit #1764

Job	Run time
benchmark-graph	47s
benchmark-ractor-only (head)	38s
benchmark-ractor (head)	6m 55s
benchmark-run-once	1m 36s
benchmark-default (ruby)	14m 19s
test (head)	1m 0s
test (truffleruby)	1m 44s
test (ruby)	45s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	13m 10s
benchmark-default (head)	13m 32s
all-tests	2s
	54m 28s

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