Pyodide 0.21.0 release

Pyodide 0.21 is a major release focused on improvements to the systems for
building and loading packages. We made it much easier to build and use binary
wheels that are not included in our distribution. We added support for Rust
packages and made major progress in simplifying support for CMake packages. We
also added 34 new packages including a large number of popular packages, for
example bitarray, opencv-python, shapely, and xgboost.