[Layout] Rename ASLayoutable to ASLayoutElement (#2290)

* [Layout API] Rename ASLayoutable to ASLayoutElement

* arg

* fix wrapperWithLayoutElement

* [Layout API] Rename ASLayoutable to ASLayoutElement

* arg

* address Michael's comments

* Remove ASLayoutValidation files that were deleted on master since this diff was created.

AsyncDisplayKit/Layout/ASLayoutElementPrivate.h

@@ -0,0 +1,95 @@
+//
+//  ASLayoutElementPrivate.h
+//  AsyncDisplayKit
+//
+//  Copyright (c) 2014-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 "ASDimension.h"
+
+@protocol ASLayoutElement;
+@class ASLayoutElementStyle;
+
+struct ASLayoutElementContext {
+  int32_t transitionID;
+  BOOL needsVisualizeNode;
+};
+
+extern int32_t const ASLayoutElementContextInvalidTransitionID;
+
+extern int32_t const ASLayoutElementContextDefaultTransitionID;
+
+extern struct ASLayoutElementContext const ASLayoutElementContextNull;
+
+extern BOOL ASLayoutElementContextIsNull(struct ASLayoutElementContext context);
+
+extern struct ASLayoutElementContext ASLayoutElementContextMake(int32_t transitionID, BOOL needsVisualizeNode);
+
+extern void ASLayoutElementSetCurrentContext(struct ASLayoutElementContext context);
+
+extern struct ASLayoutElementContext ASLayoutElementGetCurrentContext();
+
+extern void ASLayoutElementClearCurrentContext();
+
+/**
+ *  The base protocol for ASLayoutElement. Generally the methods/properties in this class do not need to be
+ *  called by the end user and are only called internally. However, there may be a case where the methods are useful.
+ */
+@protocol ASLayoutElementPrivate <NSObject>
+
+/**
+ *  @abstract This method can be used to give the user a chance to wrap an ASLayoutElement in an ASLayoutSpec 
+ *  just before it is added to a parent ASLayoutSpec. For example, if you wanted an ASTextNode that was always 
+ *  inside of an ASInsetLayoutSpec, you could subclass ASTextNode and implement finalLayoutElement so that it wraps
+ *  itself in an inset spec.
+ *
+ *  Note that any ASLayoutElement other than self that is returned MUST set isFinalLayoutElement to YES. Make sure
+ *  to do this BEFORE adding a child to the ASLayoutElement.
+ *
+ *  @return The layoutElement that will be added to the parent layout spec. Defaults to self.
+ */
+- (id<ASLayoutElement>)finalLayoutElement;
+
+/**
+ *  A flag to indicate that this ASLayoutElement was created in finalLayoutElement. This MUST be set to YES
+ *  before adding a child to this layoutElement.
+ */
+@property (nonatomic, assign) BOOL isFinalLayoutElement;
+
+@end
+
+#pragma mark - ASLayoutElementExtensibility
+
+#define ASEnvironmentLayoutExtensibilityForwarding \
+- (void)setLayoutOptionExtensionBool:(BOOL)value atIndex:(int)idx\
+{\
+  _ASEnvironmentLayoutOptionsExtensionSetBoolAtIndex(self, idx, value);\
+}\
+\
+- (BOOL)layoutOptionExtensionBoolAtIndex:(int)idx\
+{\
+  return _ASEnvironmentLayoutOptionsExtensionGetBoolAtIndex(self, idx);\
+}\
+\
+- (void)setLayoutOptionExtensionInteger:(NSInteger)value atIndex:(int)idx\
+{\
+  _ASEnvironmentLayoutOptionsExtensionSetIntegerAtIndex(self, idx, value);\
+}\
+\
+- (NSInteger)layoutOptionExtensionIntegerAtIndex:(int)idx\
+{\
+  return _ASEnvironmentLayoutOptionsExtensionGetIntegerAtIndex(self, idx);\
+}\
+\
+- (void)setLayoutOptionExtensionEdgeInsets:(UIEdgeInsets)value atIndex:(int)idx\
+{\
+  _ASEnvironmentLayoutOptionsExtensionSetEdgeInsetsAtIndex(self, idx, value);\
+}\
+\
+- (UIEdgeInsets)layoutOptionExtensionEdgeInsetsAtIndex:(int)idx\
+{\
+  return _ASEnvironmentLayoutOptionsExtensionGetEdgeInsetsAtIndex(self, idx);\
+}\