ng of digits that must appear before the decimal point
fracpart: string of digits that must come after the point
exp: exponent, as an integer
spec: dictionary resulting from parsing the format specifier

This function uses the information in spec to:
  insert separators (decimal separator and thousands separators)
  format the sign
  format the exponent
  add trailing '%' for the '%' type
  zero-pad if necessary
  fill and align if necessary
Z