Support more than one root-level Instance in rbx_dom_weak

[Dekkonot]

Currently, we only support having one root instance in a given DOM. This is great for places, but is kinda crappy for models, which can have an arbitrary amount of roots instances.

This is straightforward to implement, but is a breaking change so we'd want to plan for it, but it'd be a big win for consumers like Rojo and Lune.