Allow seek when underlying string is frozen (#121) #430

Job	Run time
ruby-versions / Generate Ruby versions	3s
ubuntu-24.04 2.7	39s
ubuntu-24.04 3.0	11s
ubuntu-24.04 3.1	12s
ubuntu-24.04 3.2	10s
ubuntu-24.04 3.3	12s
ubuntu-24.04 3.4	11s
ubuntu-24.04 debug	17s
ubuntu-24.04 head	11s
ubuntu-24.04 jruby-head	51s
ubuntu-24.04 truffleruby-head	26s
ubuntu-22.04 2.7	44s
ubuntu-22.04 3.0	11s
ubuntu-22.04 3.1	16s
ubuntu-22.04 3.2	15s
ubuntu-22.04 3.3	13s
ubuntu-22.04 3.4	10s
ubuntu-22.04 debug	16s
ubuntu-22.04 head	15s
ubuntu-22.04 jruby-head	54s
ubuntu-22.04 truffleruby-head	25s
	7m 2s

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