Added byte position range for `proc_macro::Span` Currently, the [`Debug`](https://doc.rust-lang.org/beta/proc_macro/struct.Span.html#impl-Debug-for-Span) implementation for [`proc_macro::Span`](https://doc.rust-lang.org/beta/proc_macro/struct.Span.html#) calls the debug function implemented in the trait implementation of `server::Span` for the type `Rustc` in the `rustc-expand` crate. The current implementation, of the referenced function, looks something like this: ```rust fn debug(&mut self, span: Self::Span) -> String { if self.ecx.ecfg.span_debug { format!("{:?}", span) } else { format!("{:?} bytes({}..{})", span.ctxt(), span.lo().0, span.hi().0) } } ``` It returns the byte position of the [`Span`](https://doc.rust-lang.org/beta/proc_macro/struct.Span.html#) as an interpolated string. Because this is currently the only way to get a spans position in the file, I might lead someone, who is interested in this information, to parsing this interpolated string back into a range of bytes, which I think is a very non-rusty way. The proposed `position()`, method implemented in this PR, gives the ability to directly get this info. It returns a [`std::ops::Range`](https://doc.rust-lang.org/std/ops/struct.Range.html#) wrapping the lowest and highest byte of the [`Span`](https://doc.rust-lang.org/beta/proc_macro/struct.Span.html#). I put it behind the `proc_macro_span` feature flag because many of the other functions that have a similar footprint also are annotated with it, I don't actually know if this is right. It would be great if somebody could take a look at this, thank you very much in advanced.