Use DoubleAsIfIntegral / FloatAsIfIntegral workalike in 2.13

scala.math.Numeric.DoubleAsIfIntegral doesn't exist in 2.13, and likewise for Float. As with a few other issues, it doesn't look like there's a compat library solution. One solution is to inline the 2.12 implementation for use in both versions. Another is parallel source trees again. Because it's a little tricky to make one implementation work in both 2.12/2.13 anyway, I lean towards the latter, esp. as it looks like we have to have this anyway for other issues.