Switch to vtree dependency.

VirtualNode and VirtualText were moved to the `vtree` module.
I recommend all modules depend on vtree `0.0.FIXED_NUMBER`

I'm going to blockingly upgrade everything I maintain related to vtree and vdom to 0.0.12 of those.

Since `mercury` wraps all these tools it can ensure that every copy of vtree & vdom is at the same version. 

If anyone is not using mercury they have to ensure that they have the same vtree & vdom everywhere as well.

I've noticed that 0.0.12 of virtual-dom breaks in mercury but 0.0.8 works, currently mercury depends explicitly on 0.0.8 so therefore it just works, where as if I depended on `0.0.x` it would have broken.

@Matt-Esch mentioned he's releasing 0.1.0 soon