Add a generic power function

The patch adds a `pow` function for types implementing `One`, `Mul` and
`Clone` trait.

The patch also renames f32 and f64 pow into powf in order to still have
a way to easily have float powers. It uses llvms intrinsics.

The pow implementation for all num types uses the exponentiation by
square.

Fixes bug #11499
auto merge of #11503 : FlaPer87/rust/master, r=huonw

The patch adds the missing pow method for all the implementations of the
Integer trait. This is a small addition that will most likely be
improved by the work happening in #10387.

Fixes bug #11499