Recommend wasm-snip for Rust wasm binaries
Hi,

One thing I noticed working through the tutorial is the moment you do anything that can panic e.g. `FRAMEBUFFER.as_mut().unwrap()` the binary size explodes from ~800 bytes to 27 kilobytes as Rust includes all the panicking and formatting code.

You can get a reasonable binary size by using [wasm-snip](https://github.com/rustwasm/wasm-snip) to remove that code:

In Cargo.toml
```toml
[profile.release]
opt-level = "z"
lto = true
panic = 'abort'
debug = true
```

```bash
$ cargo build --release
$ wasm-snip --snip-rust-fmt-code --snip-rust-panicking-code target/wasm32-unknown-unknown/cart.wasm
$ w4 run target/wasm32-unknown-unknown/release/cart2.wasm
```

And it seems to work as expected (note this probably means having to avoid use `std::fmt::*` stuff though, but it's probably best practice to largely avoid the standard library anyway).

It might be worth adding language speciifc notes to the docs for stuff like this,

