This patches avoids multiplying with negative powers-of-10 when parsing
floating-point values, when those powers-of-10 can be exactly represented
as a positive power.  When represented as a positive power and used to
divide, the resulting float will not have any rounding errors.

The issue is that mp_parse_num_decimal will sometimes not give the closest
floating representation of the input string.  Eg for "0.3", which can't be
represented exactly in floating point, mp_parse_num_decimal gives a
slightly high (by 1LSB) result.  This is because it computes the answer as
3 * 0.1, and since 0.1 also can't be represented exactly, multiplying by 3
multiplies up the rounding error in the 0.1.  Computing it as 3 / 10, as
now done by the change in this commit, gives an answer which is as close to
the true value of "0.3" as possible.