rectify 0.2.1

Extension methods to make Rect pleasant and convenient to work with.

[0.2.1] - 2021-03-20

• Add example.
• Export Rect, Size and Offset from dart:ui.

[0.2.0] - 2021-03-14

• Migrate to null-safety.

[0.1.0] - 2020-11-14

• Initial release.