Stop using deprecated StringSupport.memchr #592

Job	Run time
ruby-versions / Generate Ruby versions	6s
windows-latest 3.0	1m 36s
windows-latest 2.7	2m 10s
windows-latest 3.4	1m 29s
windows-latest 4.0	1m 19s
windows-latest mingw	1m 21s
windows-latest 3.1	1m 44s
windows-latest mswin	1m 32s
windows-latest 3.3	1m 20s
windows-latest jruby-head	2m 49s
windows-latest 3.2	1m 53s
	17m 19s

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