Numeric import update

Speed improvement for `double` and `float` versions of
```D
entropy
kullbackLeiblerDivergence
jensenShannonDivergence
```
OK, well... Why isn't `std.math.log2` just an alias to `core.stdc.tgmath.log2` then? I don't know the details that would lead to things being this way, but having to resort to this looks like an anti-pattern to me.