Function parse

cranelift/filetests/src/function_runner.rs:840–842

(code: &str)

    use cranelift_reader::{ParseOptions, parse_functions, parse_test};

    fn parse(code: &str) -> Function {
        parse_functions(code).unwrap().into_iter().nth(0).unwrap()
    }

    #[test]
    fn nop() {