Allow seek when underlying string is frozen (#121) #371

Job	Run time
ruby-versions / Generate Ruby versions	2s
windows-latest 2.7	3m 44s
windows-latest 3.0	1m 37s
windows-latest 3.1	2m 9s
windows-latest 3.2	2m 0s
windows-latest 3.3	3m 25s
windows-latest 3.4	1m 42s
windows-latest jruby-head	2m 13s
windows-latest mingw	1m 6s
windows-latest mswin	1m 41s
	19m 39s

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