add correct floating point min and max methods.

The `std::cmp` functions are not correct for floating point types.

`min(NaN, 2.0)` and `min(2.0, NaN)` return different values, because
these functions assume a total order. Floating point types need special
`min`, `max` and `clamp` functions.

src/libstd/num/mod.rs

@@ -313,6 +313,9 @@ pub enum FPCategory {
 pub trait Float: Signed
                + Round
                + Primitive {
+    fn max(self, other: Self) -> Self;
+    fn min(self, other: Self) -> Self;
+
     // FIXME (#5527): These should be associated constants
     fn nan() -> Self;
     fn infinity() -> Self;