Optimize `IO#read_string(0)` #9395

Job	Run time
check_format	2m 8s
check_format	2m 8s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	6m 27s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	6m 27s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	6m 28s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	6m 27s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	6m 27s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	6m 27s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	6m 27s
x86_64-gnu-test (1.7.3)	6m 26s
x86_64-gnu-test (1.7.3)	6m 26s
x86_64-gnu-test (1.8.2)	5m 55s
x86_64-gnu-test (1.8.2)	5m 55s
x86_64-gnu-test (1.9.2)	5m 37s
x86_64-gnu-test (1.9.2)	38m 9s
x86_64-gnu-test-preview_mt	7m 52s
x86_64-gnu-test-preview_mt	7m 52s
x86_64-musl-test	5m 1s
x86_64-musl-test	5m 1s
deploy_api_docs	0s
deploy_api_docs	0s
	3h 8m 56s

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