Hoist sink into selectors.

It probably makes more sense (eventually) to put it in SmallVec.

MozReview-Commit-ID: AIBKCLiMNN2

components/selectors/sink.rs

@@ -0,0 +1,31 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Small helpers to abstract over different containers.
+#![deny(missing_docs)]
+
+use smallvec::{Array, SmallVec};
+
+/// A trait to abstract over a `push` method that may be implemented for
+/// different kind of types.
+///
+/// Used to abstract over `Array`, `SmallVec` and `Vec`, and also to implement a
+/// type which `push` method does only tweak a byte when we only need to check
+/// for the presence of something.
+pub trait Push<T> {
+    /// Push a value into self.
+    fn push(&mut self, value: T);
+}
+
+impl<T> Push<T> for Vec<T> {
+    fn push(&mut self, value: T) {
+        Vec::push(self, value);
+    }
+}
+
+impl<A: Array> Push<A::Item> for SmallVec<A> {
+    fn push(&mut self, value: A::Item) {
+        SmallVec::push(self, value);
+    }
+}