Add splay GC benchmark ported from V8/WebKit #1757

Job	Run time
test (truffleruby)	1m 56s
benchmark-run-once	1m 50s
benchmark-ractor (head)	6m 46s
benchmark-ractor-only (head)	39s
benchmark-default (ruby)	14m 17s
test (ruby)	50s
test (head)	45s
benchmark-default (head)	13m 37s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	13m 51s
benchmark-graph	51s
all-tests	2s
	55m 24s

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