Reduce buffer to maximally needed size for conversion of 64bit integers.
Reduce number of used integer divisions.

benchmark            old ns/op     new ns/op     delta
BenchmarkItoa        144           119           -17.36%
BenchmarkPrintln     783           752           -3.96%

Change-Id: I6d57a7feebf90f303be5952767107302eccf4631
Reviewed-on: https://go-review.googlesource.com/2215Reviewed-by: Rob Pike <r@golang.org>