Add content rect to Fragment.

Describe box edges with extensible type.

Handle ancestor boxes in rich layout.

Add ancestor boxes to interface.

Add user data to spans.

Add line numbers to laid out fragments.

Update formatting in golden files.

All golden files are regenerated to use the new formatting.

This only affects whitespace (including newlines), otherwise the golden
values are perfectly equal. You can check this using the command:
`git show --word-diff-regex='[^[:space:]]'`

Rename golden files for easier tracking.

Make paragraphRect origin always 0,0.

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

Add ignored prefixes and suffixes to test data.

This ensures that implementation does not rely on the texts having zero
offset inside their byte arrays.

Use top edge for placing lines.

Implement absolute line heights with half-leadings.