Implement `fast_float` for `String#to_f` #3514

Job	Run time
x86_64-musl-test	4m 0s
x86_64-gnu-test (1.7.3)	1s
x86_64-gnu-test (1.8.2)	1s
x86_64-gnu-test (1.9.2)	1s
x86_64-gnu-test (1.10.1)	56s
x86_64-gnu-test (1.11.2)	47s
x86_64-gnu-test (1.12.2)	46s
x86_64-gnu-test (1.13.3)	35s
x86_64-gnu-test (1.14.0)	31s
x86_64-gnu-test-preview_mt	3m 55s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	46s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	44s
check_format	1m 39s
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
	14m 47s

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