~jaro/balkon

ref: 0680c59bcaa7239488d49b6bc9ed0aa8ae0ec3b4 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.