Add description to ASLayout (#1973)

* Add description to ASLayout

* Constrained size is a size range, print correctly

* Add missing closing bracket

AsyncDisplayKit/Layout/ASLayout.mm

@@ -10,6 +10,7 @@
 
 #import "ASLayout.h"
 
+#import "ASDimension.h"
 #import "ASInternalHelpers.h"
 #import "ASLayoutSpecUtilities.h"
 
@@ -138,6 +139,12 @@ extern BOOL CGPointIsNull(CGPoint point)
                                sublayouts:layout.sublayouts];
 }
 
+- (NSString *)description
+{
+    return [NSString stringWithFormat:@"<<ASLayout: %p>, position = %@; size = %@; constrainedSizeRange = %@>",
+            self, NSStringFromCGPoint(self.position), NSStringFromCGSize(self.size), NSStringFromASSizeRange(self.constrainedSizeRange)];
+}
+
 #pragma mark - Layout Flattening
 
 - (ASLayout *)filteredNodeLayoutTree