strconv: optimize Atoi for common case

[valyala]

Currently strconv.Atoi isn't optimized for common case when non-negative value is passed to it. Let's optimize it for this case, so users won't recommend creating custom atoi functions such as https://stackoverflow.com/a/27217267 .

The corresponding CL is coming soon.

[gopherbot]

CL https://golang.org/cl/44692 mentions this issue.