Write a demo "layout" component

To make it easier to have impressive outputs for custom data types, we should offer some common layout components. These components are similar to what we already have with `span`; they wrap a list of of child components and render them in a specific style.

To figure out how to implement this, we should pick one or two example layout components and see what we need to do to be able to write them.

The issue does not name files or tests. Start by locating the existing `span` component and reading how it wraps and renders child components; then identify what a representative layout component must support. Done means one or two example layout components can be implemented and their required component behavior is documented.

Written by the indexing model from the issue text.