Optimize children selection

[curran]

I feel like this could be improved in terms of efficiency.

...
      .selectAll(children)
      .filter(belongsToMe)
...
  function children(){
    return this.children;
  }

  function belongsToMe(){
    var instance = instanceLocal.get(this);
    return instance && instance.owner === component;
  }

The filtering could be done inside children, something like this:

  function children(){
    var mine = [];
    for(var i = 0; i < this.children.length; i++){
      if(this.children[i].tagName === tagName){ // Could check class too
        var instance = instanceLocal.get(this.children[i]);
        if(instance && instance.owner === component){
          mine.push(this)
        }
      }
    }
    return mine;
  }

[curran]

Some of the performance losses I'm seeing in the Chrome profiler may be coming from this local.get code that walks up the DOM tree: https://github.com/d3/d3-selection/blob/master/src/local.js#L15

Maybe better to just use a property on the DOM, like __instance__, as we're only using one local?