Avoid excess multiplies by multiplying in `success_probability`

Avoid excess multiplies by multiplying in `success_probability`

A substantial portion (~12%!) of our scoring time is spent dividing
the bucket pair probability by the `success_probability` divisor.

Here, we avoid this by multiplying the bucket pair probability
in floating point math and using a floating point divide (which can
be faster in some cases). This also avoids the 2^30 multiplies that
are used to avoid rounding errors when converting the float
numerator and denominator to ints.