Use `string_as_id` instead of `STRING` #1267

Job	Run time
check-misc (head)	8s
test-memory (head)	28s
steep-check (head)	13s
test-ruby (head, master)	5m 30s
test-windows (head)	2m 12s
test-ruby (3.0, master)	5m 46s
ruby-versions / Generate Ruby versions	2s
test (2.5)	24s
test (2.6)	25s
test (2.7)	28s
test (3.0)	13s
test (3.1)	13s
test (3.2)	14s
test (3.3)	11s
test (head)	12s
	16m 39s

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