Extract `readonly_string_p` #300

Job	Run time
ruby-versions / Generate Ruby versions	1m 41s
windows-latest 2.7	2m 5s
windows-latest 3.0	1m 37s
windows-latest 3.1	1m 27s
windows-latest 3.2	2m 22s
windows-latest 3.3	1m 28s
windows-latest mingw	1m 12s
windows-latest mswin	1m 7s
	12m 59s

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