The mantissa parsing code uses a floating point variable to accumulate
digits. Using an `mp_float_uint_t` variable instead and casting to
`mp_float_t` at the very end reduces code size. In some cases, it also
improves the rounding behaviour as extra digits are taken into account
by the int-to-float conversion code.
An extra test case handles the special case where mantissa overflow occurs
while processing deferred trailing zeros.
Signed-off-by: Yoctopuce dev <dev@yoctopuce.com>