Fix terminology for box layout bounds.

Add ancestor boxes to interface.

Add user data to spans.

Add line numbers to laid out fragments.

Improve terminology around fragments.

Fix documentation: references to non-imported names.

Fix style: white space in exports and imports.

Fix style: non-ASCII characters in comments.

Add internal functions for "shaped runs" output.

Update documentation and derived instances.

Use internal library to hide non-public modules.

Move non-public modules into Internal namespace.

Upgrade Fragment to contain pen position.