Separate WithSpan for sharing with other modules.

Convert plain layout to a wrapper over rich layout.

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 documentation: incorrect reference.

Integrate BiDi reordering.

Fix style: white space around operators.

Fix style: white space in exports and imports.

Remove collapsible spaces at beginning of lines.

Allow breaking at character bounds.

Add dropWhileEnd to SeparableTextContainer.

Slightly decompose the Plain module.

Move non-public modules into Internal namespace.

Remove I8/Int distinction.

The distinction was meant to provide additional type safety, but
inconsistency with other used libraries required too many workarounds
for this to be beneficial.

Minimise working with Data.Text.Internal.

Use ICU to find line break boundaries.

Implement run splitting.

Store span index in ResolvedSpan.

Implement absolute line heights with half-leadings.

Implement "plain" interface.