Allow seek when underlying string is frozen #429

Job	Run time
ruby-versions / Generate Ruby versions	2s
ubuntu-24.04 2.7	38s
ubuntu-24.04 3.0	13s
ubuntu-24.04 3.1	13s
ubuntu-24.04 3.2	14s
ubuntu-24.04 3.3	11s
ubuntu-24.04 3.4	10s
ubuntu-24.04 debug	18s
ubuntu-24.04 head	10s
ubuntu-24.04 jruby-head	57s
ubuntu-24.04 truffleruby-head	25s
ubuntu-22.04 2.7	42s
ubuntu-22.04 3.0	21s
ubuntu-22.04 3.1	18s
ubuntu-22.04 3.2	16s
ubuntu-22.04 3.3	13s
ubuntu-22.04 3.4	12s
ubuntu-22.04 debug	17s
ubuntu-22.04 head	15s
ubuntu-22.04 jruby-head	52s
ubuntu-22.04 truffleruby-head	25s
	7m 22s

Provide feedback