add cmp() to f32 and f64, with tests

Sometimes, it becomes necessary to find out if one of your floats is
bigger or smaller than another one of your floats. In my case, I had a
list of floats I had calculated, and I wanted to sort them biggest to
smallest. Using `sort_by()` requires cmp, so I initially wrote my own
fcmp, and then realized that was silly and it should just be built in.