Mark C function return values as `#[must_use]`

[briansmith]

One approach is to use (if/when it is implemented) the proposed #[must_use] annotation on functions: rust-lang/rfcs#886.

Another approach is to change the return value to a #[repr(transparent)] wrapper that is marked #[must_use], if/when this proposal is implemented: rust-lang/rfcs#1758.

This is a good-first-bug since this is something anybody can do, once the facilities are available to actually do it, but note that it requires additional work on rustc.

[briansmith]

The Rust feature #[must_use] has been implemented: rust-lang/rust#43302. The #[repr(transparent)] idea seems better to me, though; that's rust-lang/rust#43036.