Implement `fast_float` for `String#to_f` #3513

Job	Run time
check_format	1m 38s
x86_64-gnu-test (1.7.3)	1s
x86_64-gnu-test-preview_mt	5m 39s
x86_64-musl-test	13m 2s
x86_64-gnu-test (1.8.2)	1s
x86_64-gnu-test (1.9.2)	1s
x86_64-gnu-test (1.10.1)	1m 1s
x86_64-gnu-test (1.11.2)	39s
x86_64-gnu-test (1.12.2)	39s
x86_64-gnu-test (1.13.3)	33s
x86_64-gnu-test (1.14.0)	1s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	45s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	40s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	1s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	1s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	1s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	1s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	1s
	24m 45s

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