This patch fixes 2 things when printing a floating-point number that
requires rounding up of the mantissa:
- retain the correct precision; eg 0.99 becomes 1.0, not 1.00
- if the exponent goes from -1 to 0 then render it as +0, not -0
C's printf will pad nan/inf differently to CPython. Our implementation
originally conformed to C, now it conforms to CPython's way.
Tests for this are also added in this patch.
