~jaro/balkon

ref: 2d06bc9a995587a140a440f89cb37923fcb7f46a balkon/lib/Data 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.