Change span of as_dyn_error() to point compile error at attribute.

[de-vri-es]

This PR changes the span of the as_dyn_error() call to the #[source], #[from] or #[transparant] attribute if it exists.

My motivation was a (to me) confusing error that actually made me think thiserror had a bug:

use thiserror::Error;

#[derive(Debug, Error)]
#[error("{0}")]
enum Error {
    Io(#[from] std::io::Error),
    Custom(#[from] String),
}

The error uses the span of the source member, but in this case it is unnamed. How exactly it ends up with the span of the string literal is a bit unclear to me. Maybe because the member (0) is used in the formatting code?

Regardless, with this PR, the error is the same but the span is changed to the attribute:

If there was no #[source], #[from] or #[transparent] attribute, the member name is used as before (which in practise means it must be a member named source).

[dtolnay]

Thanks!