Add #[warn_unused_result] attribute for functions

[bblum]

Just like __attribute__((warn_unused_result)) in C.

One thing this would be really useful for is to help people distinguish functional-style vs in-place-style data structure update functions. I could have saved some time during ICFP (#2928) 'cause I was using vec::append as though it were vec::push_all.

Marking this I-completion because I care a lot about it. Feel free to debate its importance.

[nikomatsakis]

@pcwalton wants (and I tend to agree) to make all expressions warn if they have an unused result, rather like O'Caml.

[catamorphism]

Changing to RFC so we can discuss "make all expressions warn if they have an unused result." I support that as well.

[nikomatsakis]

Created a separate issue (#2974) for a more general version of this. Closing for now in favorite of that issue.