Add support for wasm32-unknown-emscripten

[messense]

Status:

• Add support for wasm32-unknown-emscripten target #974
• What about wasm32-wasi?

Upstream:

• Python WASM support: https://pythondev.readthedocs.io/wasm.html
• Building a Python package that uses Rust pyodide/pyodide#1973

[messense]

Let's worry about wasm32-wasi when Python gains better support for it.