Optimize `IO#read_string(0)` #9401

Job	Run time
deploy_api_docs	0s
check_format	2m 20s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	36m 52s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	42m 17s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	45m 48s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	37m 49s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	54m 36s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	38m 39s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	46m 33s
x86_64-gnu-test (1.7.3)	36m 12s
x86_64-gnu-test (1.8.2)	36m 6s
x86_64-gnu-test (1.9.2)	36m 40s
x86_64-musl-test	43m 7s
x86_64-gnu-test-preview_mt	7m 30s
	7h 44m 29s

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