Generalise shiftFragment function.

Intended for testing paragraphSafeWidth.

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.