add a lint check for unused pure function return values

[thestinger]

If the definition of pure is changed to allow mutating arguments, functions with &mut arguments could just be ignored by this check. Alternatively, if there ends up being something like mut pure as a lighter form of purity, those ones would be skipped.

[catamorphism]

+1

[pcwalton]

I think this should extend to all function return values, actually, except those for C functions or those notated by #[ignorable_return_value] (or something like that).

[graydon]

pure functions are going away, so closing this as WONTFIX (#2974 is the general case)