~jaro/balkon

ref: 65d952e2f615751db54267097c706545be5e6be6 balkon/lib d---------
Slightly decompose the Plain module.
Update documentation and derived instances.
Expose ParagraphConstruction.
Hide Rect union from public interface.

The function behaves in a specific way that should not be relied on by
other modules.
Use internal library to hide non-public modules.