Compile on stable Rust 2018 #13

It would be neat to be able to use this on stable (once use_extern_macros is stabilized). Currently the following features are activated:

• #![feature(in_band_lifetimes)]: tracking issue, expected to be stabilized for Rust 2018. If not, we can easily remove the feature by changing one line of code. => probably won't be stabilized, so this has been removed from this crate for now.
• #![feature(proc_macro_span)]: used only for one Span::join() call. I think it's easy to remove join() with something we can use on stable. => guarded by nightly feature (see Guard unstable proc_macro things behind nightly feature #26)
• #![feature(proc_macro_diagnostic)]: this is the big part! I don't think the Diagnostics API will be stabilized in near future. So in order to compile on stable, we need to replace all nice diagnostics with simple panics. This shouldn't be too hard. => guarded by nightly feature (see Guard unstable proc_macro things behind nightly feature #26)

We still use Cargo's cargo-features = ["edition"] (EDIT: it's stabilized on nightly and beta RC1 now) with edition ="2018" which entails a couple of features. But all of these should be stabilized for Rust 2018 (EDIT: they are now).

• Implicit extern crates (see (Modules) Tracking issue for (use) crate_name:: paths without extern crate rust-lang/rust#53128)
• #![feature(extern_prelude)] (see (Modules) Tracking issue for (use) crate_name:: paths without extern crate rust-lang/rust#53128)
• #![feature(crate_visibility_modifier)] (see (Modules) Tracking issue for crate as a visibility modifier rust-lang/rust#53120) (TODO: this might not make it into Rust 2018, maybe switch to pub(crate) for now?) -> replaced by pub(crate)
• #![feature(crate_in_paths)] (see (Modules) Tracking issue for Picking a Module Path System variant rust-lang/rust#53130)