rust: ErrorContext ownership isn't modeled in APIs

This input:

```wit
package test:foo;

world the-world {
  import x: func(x: list<error-context>);
}
```

generates:

```rust
pub fn x(x: &[wit_bindgen::rt::async_support::ErrorContext]) -> () {
```

but I believe that error-context values are passed by ownership, so this semantically isn't the right signature for Rust.