Fix the access to the renderer from every node

[smaramba]

This pull request fix the access to the renderer from every node.
In the previous implementation the renderer computed property of the AnyNode protocol was erased by the renderer variable of the Node class.

Solution: store the reference of the renderer inside a private _renderer: Renderer? variable in Node, expose a computed property renderer: Renderer from AnyNode.

public class Renderer {
}

public class Node<T> {
  private var _renderer: Renderer?
  public var parent: AnyNode?
  
  public var renderer: Renderer {
    guard self._renderer == nil else { return self._renderer! }
    return self.parent!.renderer
  }
}

public protocol AnyNode: class {
  var renderer: Renderer { get }
  var parent: AnyNode? { get }
}

protocol InternalAnyNode: AnyNode {}

extension Node: InternalAnyNode {}