big: refine printf formatting and optimize string conversion

big: refine printf formatting and optimize string conversion

Now handles standard precision specifications, standard interactions of
redundant specifications (such as precision and zero-fill), handles the
special case of precision specified but equal to zero, and generates the
output without recursive calls to format/printf to be clearer and faster.

R=gri, mtj, gri
CC=golang-dev
https://golang.org/cl/4703050