Introcs

* Improved speed 0 performance on macOS for complex recursive shapes

* Added HSL to the color models for a new activity

* Fixed quit_with_error to raise SystemExit() rather than call quit()
* Fixed an error with Unicode support in urltools

* Fixed cursor positioning in window turtle

* Fixed stray character that had somehow entered Turtle code
* Added turtle test cases to prevent this happening in future

* Added `assert_error` to `testcase` for checking function asserts
* Added code rewriting to `modlib` (still unexposed) to guard against infinite loops
* Modified `Environment` in `modlib` to load from file or source string
* Added feature to `Environment` to execute code as a script (`__name__ == '__main__'`)