Inline functions Ordering::{then, then_with}

@jongiddy noticed bad performance due to the lack of inlining on `then`
and `then_with`. I confirmed that inlining really is the culprit by
creating a custom `then` function and repeating his benchmark on my
machine with and without the `#[inline]` attribute.

The numbers were exactly the same on my machine without the attribute.
With `#[inline]` I got the same performance as I did with manually
inlined implementation.

Author: Stjepan Glavina

src/libcore/cmp.rs

@@ -277,6 +277,7 @@ impl Ordering {
     ///
     /// assert_eq!(result, Ordering::Less);
     /// ```
+    #[inline]
     #[unstable(feature = "ordering_chaining", issue = "37053")]
     pub fn then(self, other: Ordering) -> Ordering {
         match self {
@@ -315,6 +316,7 @@ impl Ordering {
     ///
     /// assert_eq!(result, Ordering::Less);
     /// ```
+    #[inline]
     #[unstable(feature = "ordering_chaining", issue = "37053")]
     pub fn then_with<F: FnOnce() -> Ordering>(self, f: F) -> Ordering {
         match self {