Formats given decimal digits
0.<...buf...> * 10^exp into the decimal form
with at least given number of fractional digits. The result is stored to
the supplied parts array and a slice of written parts is returned.
frac_digits can be less than the number of actual fractional digits in
it will be ignored and full digits will be printed. It is only used to print
additional zeroes after rendered digits. Thus
frac_digits of 0 means that
it will only print given digits and nothing else.