Link ResolvedSpan to boxes.

Embed TextOptions in ResolvedSpan.

SpanOptions are relegated to the legacy Plain interface, and
TextOptions are now being used internally.

Add user data to spans.

Fix style: external vs internal imports.

Fix style: white space in exports and imports.

Fix test warnings.

Move non-public modules into Internal namespace.

Minimise working with Data.Text.Internal.

Use ICU to find line break boundaries.

Store span index in ResolvedSpan.

Implement absolute line heights with half-leadings.

Implement "plain" interface.

Separate tests by module.