[WIP] Add support for nextSibling for partial rehydration

[chiragpat]

Putting up a draft PR for supporting nextSibling use cases for partial rehydration. I am not sure we want to do this before a refactoring of element builders but putting up a draft for future reference.

Why
With partial rehydration we need to be able to let the rehydrate builder which particular component in the container is being rehydrated. This can be achieved by passing a nextSibling in the initial cursor to the renderComponent call

How

• Modify rehydrate builder to walk backwards from the nextSibling to find the first closing block
• Then walk backwards from there to find the first opening block at the same depth

[NullVoxPopuli]

Is this still worth pursuing?