Add Aggregate.elementsIterator and micro-optimize

elementsIterator provides a more efficient API for iterating on the
elements of Aggregates. It is especially useful for Records where
getElements returns a Seq and thus eagerly constructs a new
datastructure which may then just be iterated on anyway.

This new elementsIterator API is then used throughout the codebase where
it makes sense.

Also change Vec.getElements to just return the underlying self instead
of constructing a new Seq.