[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!

[SirMishaa]

Working for this one to be merged too, any plans ? 👀

[onlycs]

I'm gonna resolve the merge conflicts at some point but this should be working on the latest version soon™

I have like, negative free time rn.

[onlycs]

this is up-to-date and ready. all tests completed successfully

@dtolnay can this merge?