To build `demo.wasm` use rustc (nightly) for wasm32 target with debug information:
```
rustc +nightly --target=wasm32-unknown-unknown demo.rs --crate-type=cdylib
```
Point path to the built `wasmtime_py` library location when running python, e.g.
```
PYTHONPATH=../../target/debug python3 run.py
```