Fix style: white space in exports and imports.

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.

Make paragraphRect origin always 0,0.

This convention should make it easier for the caller to properly place
the paragraph within a document.

Fix mistakes in documentation comments.

Add module for working with rectangle coordinates.