Add .retain() method

Based on "retain_mut" as discussed for vec: we can provide &mut to the
element to remove effortlessly.

src/lib.rs

@@ -277,6 +277,40 @@ impl<A: Array> ArrayVec<A> {
         while let Some(_) = self.pop() { }
     }
 
+    /// Retains only the elements specified by the predicate.
+    ///
+    /// In other words, remove all elements `e` such that `f(&mut e)` returns false.
+    /// This method operates in place and preserves the order of the retained
+    /// elements.
+    ///
+    /// ```
+    /// use arrayvec::ArrayVec;
+    ///
+    /// let mut array = ArrayVec::from([1, 2, 3, 4]);
+    /// array.retain(|x| *x & 1 != 0 );
+    /// assert_eq!(&array[..], &[1, 3]);
+    /// ```
+    pub fn retain<F>(&mut self, mut f: F)
+        where F: FnMut(&mut A::Item) -> bool
+    {
+        let len = self.len();
+        let mut del = 0;
+        {
+            let v = &mut **self;
+
+            for i in 0..len {
+                if !f(&mut v[i]) {
+                    del += 1;
+                } else if del > 0 {
+                    v.swap(i - del, i);
+                }
+            }
+        }
+        if del > 0 {
+            self.drain(len - del..);
+        }
+    }
+
     /// Set the vector's length without dropping or moving out elements
     ///
     /// May panic if `length` is greater than the capacity.