~jaro/balkon

ref: 2a2cdd51f9c7560ff2eb6e92a2fe3a4b75b54e1c 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.