Implement proposed API for `proc_macro_span` As proposed in [#54725 (comment)](https://github.com/rust-lang/rust/issues/54725#issuecomment-1546918161). I have omitted the byte-level API as it's already available as [`Span::byte_range`](https://doc.rust-lang.org/nightly/proc_macro/struct.Span.html#method.byte_range). `@rustbot` label +A-proc-macros r? `@m-ou-se`