Fabric: Using ShadowView instead of ShadowNode in Mutations

Summary:
@public
This is quite a big diff but the actual meaningful change is simple: now we use ShadowView class instead of ShadowNode in mutation instructions.
Note:
 * In some places (especially during diffing) we have to operate with ShadowNodeViewPair objects (which represents a pair of ShadowNode and ShadowView). The reason for that is that we cannot construct child ShadowViews from parent ShadowViews because they don't have any information about children.
 * `ShadowTree::emitLayoutEvents` is now much simpler because ShadowView better represents the specifics of this kind of object.
 * The code in RCTMountingManager also became simpler.

This change will allow us to implement more cool tricks soon.

Reviewed By: mdvacca

Differential Revision: D9403564

fbshipit-source-id: dbc7c61af250144d6c7335a01dc30df0005559a2