Add support for native animations on iOS

Summary:
Currently on iOS animations are being performed on the JS thread. This ports animations over to the native thread and performs them natively. A lot of this work has already been done on Android, but this PR enables a few animation nodes that Android doesn't yet support such as Transform, Multiplication, and Addition nodes.
Also there is a demo of the native animations added to the UIExplorer app.
Closes https://github.com/facebook/react-native/pull/7884

Differential Revision: D3401811

Pulled By: nicklockwood

fbshipit-source-id: c8d750b75e4410923e17eaeb6dcaf079a09942e2

Libraries/NativeAnimation/Nodes/RCTAdditionAnimatedNode.m

@@ -0,0 +1,27 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import "RCTAdditionAnimatedNode.h"
+
+@implementation RCTAdditionAnimatedNode
+
+- (void)performUpdate
+{
+  [super performUpdate];
+  NSArray<NSNumber *> *inputNodes = self.config[@"input"];
+  if (inputNodes.count > 1) {
+    RCTValueAnimatedNode *parent1 = (RCTValueAnimatedNode *)self.parentNodes[inputNodes[0]];
+    RCTValueAnimatedNode *parent2 = (RCTValueAnimatedNode *)self.parentNodes[inputNodes[1]];
+    if (parent1 && parent2) {
+      self.value = parent1.value + parent2.value;
+    }
+  }
+}
+
+@end