~jaro/balkon

ref: d5807a8ebe4763c4fbdf157357e16eae70ff0f00 balkon/src/Data/Text/ParagraphLayout/Internal d---------
Add trivial instances of text containers.
Fix documentation of ProtoFragment after RTL placement.
Implement correct positioning of RTL runs.
Make SpanOptions a separate type.
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.