Test forming and breaking ligatures.

Allow breaking at character bounds.

Ensure paragraph rect is unaffected by spans.

Insert extra test to work around segmentation fault.

Without the added test, the test suite causes a segmentation fault
after starting "wraps by characters when line is ultra narrow".

Add line wrapping tests with spans.

Move non-public modules into Internal namespace.

Implement "plain" interface.

Separate tests by module.