~jaro/balkon

ref: 35a5cc75e3831fa3139ebb43dcf2e0948a76be16 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.