Initial implementation of element merge strategy

This commit supports attribute merging in component invocations by
storing the attribute operations for an element in
GroupedElementOperations. In `CloseElementOpcode`, we merge all of the
seen operations and execute them.

In principle, this work is only needed for component elements, which can
have multiple sources of attributes with arbitrary merge strategies.
This commit adds the GroupedElementOperations to every element, which
mildly regresses performance.

We plan to limit the impact to component elements, and have plans to
avoid the work in all but the most dynamic scenarios. That said, this
works and passes a new somewhat aggressive smoke test based on one that
helped us understand the nature of the problem in Glimmer 1.

Near-term work:

* Avoid GroupedElementOperations for regular elements
* Avoid emitting UpdatingOpcodes for static attributes on components
* Extract merge strategy into ComponentManager (and possibly allow
  kinds of attributes to control their own merging directly)
* Correctly group the operations so that invoker and invokee attributes
  can be properly interpreted by the merging logic.