对于负数来说，是不太合适将他们转换为二进制、八进制和十六进制的，但是因为某些工程实用性的原因，fast_itoa通过取绝对值转换并添加负号来实现转换。
For negative numbers, it is illegal to convert them to binary, octal or hexadecimal form.
But for some practical reasons, fast_itoa can deal with them by adding - to the converted result of their absolute value.
Environment: Intel(R) Core(TM) i7-4710MQ CPU @ 2.50GHz / Win64 / GCC_7.1.0@MinGW-w64
Count: 100000000
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itoa2: 506 ms
(C)itoa: 20385 ms
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itoa8: 608 ms
snprintf: 6088 ms
(C)itoa: 5515 ms
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itoa10: 540 ms
snprintf: 5623 ms
(C)itoa: 4803 ms
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itoa16: 271 ms
snprintf: 5402 ms
(C)itoa: 3878 ms
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