Fixes so the compiler can infer msg_send! return types (backport to 0.18) This is a backport of #340 onto the 0.18 release. Currently, due to a quirk in Rust's type inference interacting with the structure of the `msg_send!` macro, a `()` return type will be inferred when the compiler cannot otherwise determine the return type. This behavior is expected to change, and in the future could resolve to a `!` return type, which results in undefined behavior. Linting has previously been added for this in rust-lang/rust#39216, but it did not catch these cases due to SSheldon/rust-objc#62. An upcoming version of objc will be fixed to stop hiding these errors, at which point they will become compile errors. This change fixes these errors and allows cocoa to compile with the fixed version of objc.