Add `core::panic::Location::{end_line,end_column}`?

Method chains no longer share a single span, which makes the location reported by `Location::caller()` *much* more useful when chaining unwraps. The next step to improve their fidelity will be to allow `Location` to encode the "end span" of a call, and to expose that to users somehow. Probably in the Debug/Display impls to start.