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/RCTAnimatedNode.h

@@ -0,0 +1,54 @@
+/**
+ * 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 <Foundation/Foundation.h>
+
+@interface RCTAnimatedNode : NSObject
+
+- (instancetype)initWithTag:(NSNumber *)tag
+                     config:(NSDictionary<NSString *, id> *)config NS_DESIGNATED_INITIALIZER;
+
+@property (nonatomic, readonly) NSNumber *nodeTag;
+@property (nonatomic, copy, readonly) NSDictionary<NSString *, id> *config;
+
+@property (nonatomic, copy, readonly) NSDictionary<NSNumber *, RCTAnimatedNode *> *childNodes;
+@property (nonatomic, copy, readonly) NSDictionary<NSNumber *, RCTAnimatedNode *> *parentNodes;
+
+@property (nonatomic, readonly) BOOL needsUpdate;
+@property (nonatomic, readonly) BOOL hasUpdated;
+
+/**
+ * Marks a node and its children as needing update.
+ */
+- (void)setNeedsUpdate NS_REQUIRES_SUPER;
+
+/**
+ * The node will update its value if necesarry and only after its parents have updated.
+ */
+- (void)updateNodeIfNecessary NS_REQUIRES_SUPER;
+
+/**
+ * Where the actual update code lives. Called internally from updateNodeIfNecessary
+ */
+- (void)performUpdate NS_REQUIRES_SUPER;
+
+/**
+ * Cleans up after a round of updates.
+ */
+- (void)cleanupAnimationUpdate NS_REQUIRES_SUPER;
+
+- (void)addChild:(RCTAnimatedNode *)child NS_REQUIRES_SUPER;
+- (void)removeChild:(RCTAnimatedNode *)child NS_REQUIRES_SUPER;
+
+- (void)onAttachedToNode:(RCTAnimatedNode *)parent NS_REQUIRES_SUPER;
+- (void)onDetachedFromNode:(RCTAnimatedNode *)parent NS_REQUIRES_SUPER;
+
+- (void)detachNode NS_REQUIRES_SUPER;
+
+@end