Add code coverage to PyWebAssembly

[axic]

PyWebAssembly follows the wasm spec very closely, so by adding code coverage to it and running only the official wasm test suite should give very accurate report of official test coverage.

[poemm]

It seems that coverage.py is popular. I will look into this.

[chfast]

Yes, use coverage package to get coverage report. Then upload to Codecov with codecov package.

[chfast]

https://github.com/codecov/example-python