Allow seek when underlying string is frozen #426

Job	Run time
ruby-versions / Generate Ruby versions	6s
ubuntu-24.04 2.7	43s
ubuntu-24.04 3.0	11s
ubuntu-24.04 3.1	11s
ubuntu-24.04 3.2	24s
ubuntu-24.04 3.3	14s
ubuntu-24.04 3.4	16s
ubuntu-24.04 debug	18s
ubuntu-24.04 head	16s
ubuntu-24.04 jruby-head	51s
ubuntu-24.04 truffleruby-head	26s
ubuntu-22.04 2.7	38s
ubuntu-22.04 3.0	32s
ubuntu-22.04 3.1	14s
ubuntu-22.04 3.2	14s
ubuntu-22.04 3.3	19s
ubuntu-22.04 3.4	11s
ubuntu-22.04 debug	18s
ubuntu-22.04 head	11s
ubuntu-22.04 jruby-head	58s
ubuntu-22.04 truffleruby-head	26s
	7m 57s

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