Implement run splitting.

Lay out Runs independently of Spans.

Implement absolute line heights with half-leadings.

Upgrade Fragment to contain pen position.

Use infix operators for constructing test data.

Implement "plain" interface.

Create "plain" interface.

Add module for working with rectangle coordinates.

Clean up and update package description file.

Automatically discover test specs.

Separate tests by module.

Add unit tests for spanToRuns.

Separate modules.

WIP version with WIP interface.