Lifetime elision in impl

Minimum Rust version: nightly

When writing an impl, you can mention lifetimes without them being bound in the argument list.

In Rust 2015:

impl<'a> Iterator for MyIter<'a> { ... }
impl<'a, 'b> SomeTrait<'a> for SomeType<'a, 'b> { ... }

In Rust 2018:

impl Iterator for MyIter<'iter> { ... }
impl SomeTrait<'tcx> for SomeType<'tcx, 'gcx> { ... }