a_0.2.0

* Added more examples with a common framework

* changed build script to download assets from github
* added script that downloads assets from flutter in github workflow. uses git submoduele's commit as the source of truth for engine SHA
* made sys module's visibility behind a feature. It adds too much noise to autocomplete.
* polished codegen to add ignore after backticks in doc comments because they were causing cargo test to fail.
* fixed some methods being associated functions (was I asleep when writing them?)
* made every handle method take &self instead of &mut self for convenience
* added color constants by parsing a json file from github.
* take color/point by value instead of reference, as they are copy and you can avoid & everywhere.
* more docs.