Deduplicate emptyFont in RunSpec.

Generalise text from SpanData for other tests.

Add Arabic examples to spanToRuns tests.

Fix style: white space in exports and imports.

Reduce test dependencies on fonts.

Move non-public modules into Internal namespace.

Minimise working with Data.Text.Internal.

Link to Span using a generic wrapper.

Implement "plain" interface.

Separate tests by module.