rust: ErrorContext ownership isn't modeled in APIs

[alexcrichton]

This input:

package test:foo;

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

generates:

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.