Use `string_as_id?` #1415

Job	Run time
steep-check (head)	18s
check-misc (head)	7s
test-memory (head)	36s
test-windows (head)	1m 48s
test-ruby (head, master)	6m 3s
test-ruby (3.0, master)	6m 57s
ruby-versions / Generate Ruby versions	2s
test-cpp (head, g++)	17s
test (2.5)	29s
test-cpp (head, clang++)	19s
test (2.6)	30s
test (2.7)	28s
test (3.0)	12s
test (3.1)	24s
test (3.2)	19s
test (3.3)	12s
test (head)	11s
	19m 12s

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