Microbenchmark for rb_sym2str under Ractors #1946

Job	Run time
benchmark-run-once	1m 41s
benchmark-default (ruby)	11m 35s
benchmark-ractor-only (head)	50s
benchmark-default (head)	13m 7s
benchmark-graph	39s
benchmark-ractor (head)	6m 57s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	12m 34s
test (truffleruby)	1m 26s
test (ruby)	44s
test (head)	41s
all-tests	2s
	50m 16s

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