Speed up integer parsing

[chriso]

There's no need to check for possible overflow before we handle each byte of input. We can just handle the next byte and then check whether overflow occurred. This shrinks the overflow checks in half.

amd64:

name old time/op new time/op delta Unmarshal/*json.codeResponse2-4 4.94ms ± 2% 4.74ms ± 1% -4.11% (p=0.008 n=5+5) name old speed new speed delta Unmarshal/*json.codeResponse2-4 393MB/s ± 2% 409MB/s ± 1% +4.27% (p=0.008 n=5+5) 

arm64:

name old time/op new time/op delta Unmarshal/*json.codeResponse2-10 4.43ms ± 0% 4.26ms ± 0% -3.82% (p=0.000 n=19+18) name old speed new speed delta Unmarshal/*json.codeResponse2-10 438MB/s ± 0% 455MB/s ± 0% +3.97% (p=0.000 n=19+18) 

[achille-roussel]

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