Rewrite render() to use NodePart

[justinfagnani]

This supersedes #445

Fixes #422

[ruphin]

I was looking into this exact patch yesterday. I came up with an implementation very similar to this, although yours looks a bit cleaner. The biggest problem with this change is that each render(<something>) now injects two markers that it didn't need before, which affects performance.

I think it should be possible to use a single marker. A NodePart only needs one node to exist (before or after) so it can reference it's parent. If the other node doesn't exist, it can assume that it is the first node (or the last node) in the context. This will require some refactoring of parts in general, but I feel like the number of required markers can be cut down, which will not only affect render(), but other instances of parts as well. My own implementation of NodePart does exactly this, and although it adds a bit of complexity, it creates one less marker for most instances of NodePart.

[justinfagnani]

@ruphin there are actually a number of places where we can reduce markers. In this case (and a few others) we should be able to get it down to 0 markers. I think I want to get this in and attack markers more holistically.