Provide an API to access byte offsets for Spans

[buchgr]

In Rust Nightly proc_macro::Span has a method byte_range(). Having access to byte offsets in addition to (line, column) information is useful to some applications, including my own. I understand that byte_range() is currently only available in the nightly version, but proc_macro2::Span could expose byte offsets today when used in a main.rs context.

David, would you accept a PR that exposes byte offsets via proc_macro2::Span?