~jaro/balkon

ref: ba1e5a040d130d35a16a7d9a0146cec41262cbff balkon/lib/Data/Text 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.