Tracking issue for RFC 1940: allow `#[must_use]` on functions

[aturon]

This is a tracking issue for the RFC "allow #[must_use] on functions" (rust-lang/rfcs#1940).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

[zackmdavis]

I think I should have an implementation shortly.