Rely on Collection conformance for .first and .last

There's no need to go through _elements.

Author: ole

Sources/SortedArray.swift

@@ -133,15 +133,15 @@ extension SortedArray {
     /// - Complexity: O(1).
     @warn_unqualified_access
     public func min() -> Element? {
-        return _elements.first
+        return first
     }
 
     /// Returns the maximum element in the sequence.
     ///
     /// - Complexity: O(1).
     @warn_unqualified_access
     public func max() -> Element? {
-        return _elements.last
+        return last
     }
 }