Update src/string.cr #2723

Job	Run time
x86_64-gnu-test (1.7.3)	35m 35s
x86_64-gnu-test (1.8.2)	35m 25s
x86_64-gnu-test (1.9.2)	38m 41s
x86_64-gnu-test (1.10.1)	34m 37s
x86_64-gnu-test (1.11.2)	35m 16s
x86_64-gnu-test-preview_mt	5m 41s
x86_64-gnu-test (1.12.2)	35m 27s
x86_64-gnu-test (1.13.3)	32m 57s
x86_64-gnu-test (1.14.0)	35m 28s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	34m 47s
x86_64-musl-test	37m 59s
check_format	1m 49s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	35m 30s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	35m 56s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	34m 18s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	33m 14s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 45s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	32m 39s
	9h 30m 4s

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