potentially missing items from rust_2018_edition feature gate

[nikomatsakis]

I noticed recently some feature gates that rust_2018_edition does not enable that I think it should:

• #![feature(catch_expr)]
• #![feature(crate_in_paths)]
• #![feature(crate_visibility_modifier)]
• #![feature(extern_absolute_paths)]
• #![feature(proc_macro)]
• #![feature(in_band_lifetimes)]
• #![feature(termination_trait_test)]

cc @Manishearth

[Manishearth]

Most of these features seem to be enabled already? It's rust_2018_preview. It's only catch_expr and proc_macro that aren't.