Consider adding #[must_use] annotation to some types, functions, and macros

[joshlf]

The functions that probably need this the most are the write_to_xxx methods. These return an Option<()>, and so it'd be easy to call write_to_xxx, ignore this return value, and assume that the write had unconditionally succeeded (or assume that, as is often the case in Rust APIs, the method will panic on failure.

We should also use #[must_use] in transmute!, transmute_ref!, and transmute_mut!, as they are side effect-free.

See also #284