Allow seek when underlying string is frozen #367

Job	Run time
ruby-versions / Generate Ruby versions	17s
windows-latest 2.7	2m 59s
windows-latest 3.0	1m 31s
windows-latest 3.1	1m 5s
windows-latest 3.2	1m 52s
windows-latest 3.3	1m 41s
windows-latest 3.4	1m 56s
windows-latest jruby-head	1m 46s
windows-latest mingw	1m 27s
windows-latest mswin	1m 39s
	16m 13s

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