Extract `readonly_string_p` #359

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	35s
ubuntu-22.04 3.0	13s
ubuntu-22.04 3.1	11s
ubuntu-22.04 3.2	15s
ubuntu-22.04 3.3	12s
ubuntu-22.04 debug	21s
ubuntu-22.04 head	11s
ubuntu-22.04 jruby-head	57s
ubuntu-22.04 truffleruby-head	25s
ubuntu-20.04 2.7	37s
ubuntu-20.04 3.0	9s
ubuntu-20.04 3.1	13s
ubuntu-20.04 3.2	14s
ubuntu-20.04 3.3	14s
ubuntu-20.04 debug	24s
ubuntu-20.04 head	14s
ubuntu-20.04 jruby-head	56s
ubuntu-20.04 truffleruby-head	23s
	6m 47s

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