cmp: switch min and max to TotalOrd

The `Float` trait provides correct `min` and `max` methods on floating
point types, providing a consistent result regardless of the order the
parameters are passed.

These generic functions do not take the necessary performance hit to
correctly support a partial order, so the true requirement should be
given as a type bound.

Closes #12712

src/doc/guide-tasks.md

@@ -353,8 +353,6 @@ fn pnorm(nums: &~[f64], p: uint) -> f64 {
 
 fn main() {
     let numbers = vec::from_fn(1000000, |_| rand::random::<f64>());
-    println!("Inf-norm = {}",  *numbers.iter().max().unwrap());
-
     let numbers_arc = Arc::new(numbers);
 
     for num in range(1u, 10) {