`root` and `pow` don't handle `Int` arguments quite right

At present, the implementations of these operations convert their `Int` arguments to the floating-point type. This works fine for small integers (the common case), but isn't quite right for large integers (which will round on conversion). This is especially problematic because it can convert odd integers to even integers, which will change the sign of results when the floating-point argument is negative.

Locate the implementations of root and pow that accept Int arguments, then inspect how those arguments are converted to floating point. Check behavior with large odd integers and negative floating-point arguments; done means those cases no longer round in a way that changes the result.

Written by the indexing model from the issue text.