Lifetime elision in trait constraints

[steveklabnik]

Originally from rust-lang/rust#13840:

This compiles:

fn isum<'a, I: Iterator<&'a i32>>(mut it: I) -> int { it.fold(0, |a, b| a + *b) }

This should be the same, but doesn't compile:

fn isum<I: Iterator<&i32>>(mut it: I) -> i32 { it.fold(0, |a, b| a + *b)}

The compiler complains of a "missing lifetime specifier" at the &int.