stdlib: add a hook for dynamic dispatch in CollectionType.find()

This hook allows Set.find() to be equally efficient in static and
generic contexts.

This time, with correct tests.

Swift SVN r27404

stdlib/public/core/CollectionAlgorithms.swift.gyb

@@ -49,9 +49,11 @@ extension CollectionType where Self.${GElement} : Equatable {
   ///
   /// Complexity: O(\ `self.count()`\ )
   final public func _prext_find(element: ${GElement}) -> Index? {
-    // FIXME: dynamic dispatch for Set and Dictionary.
-    // FIXME: _prext_indices
-    for i in indices(self) {
+    if let result? = _customFindEquatableElement(element) {
+      return result
+    }
+
+    for i in self._prext_indices {
       if self[i] == element {
         return i
       }