Add lazy list/vstack/hstack

With the new infrastructure for dynamic views in place (see e.g. #31, #32) these should be reasonably easy to implement.

Start by reading the dynamic-view infrastructure referenced in issues #31 and #32, then locate the existing list, VStack, and HStack entry points. Define the scope of their lazy variants from the surrounding implementation; the work is done when all three components are implemented and validated by the repository's relevant checks.

Written by the indexing model from the issue text.