~jaro/balkon

ref: 07b8b398f5731c2119314beafcec063e4ac33303 balkon/src/Data/Text/ParagraphLayout/Internal 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.
Move non-public modules into Internal namespace.