[Rust] cannot allocate a String in update
Hi,

I'm having trouble allocating a `String`, am I missing something?

```rust
// lib.rs
#[no_mangle]
fn update() {
    let mut my_string = String::new();

    my_string.push('#');

    text(&my_string.clone(), 20, 20);
}
```

```
Uncaught (in promise) RuntimeError: unreachable
    at update (<anonymous>:wasm-function[11]:0x20fb)
    at p.update (wasm4.js:1)
    at t (wasm4.js:1)
    at wasm4.js:1
    ```