[Feature] std::panic::Location support

[onlycs]

closes #142

#[derive(Debug, Error)]
struct MyError {
  #[from] err: SomeOtherError,
  #[location] /*optional attr*/ location: &'static Location<'static>,
}

[mpalmer]

This is a solid change. I've started using it in a project of mine and it's working great.

[juntyr]

+1

[ishbir]

Anything blocking the merge of this? It would be a great feature to have!