feat: impl std::error::Error for Frame

[andylokandy]

This refers to #20

[andylokandy]

Seems not possible to override and proxy the downcast_ref of dyn std::error::Error to the inner error, so downcast will not work once Exn is wrapped into a dyn Error.

[andylokandy]

Seems not possible to override and proxy the downcast_ref of dyn std::error::Error to the inner error, so downcast will not work once Exn is wrapped into a dyn Error.

anyhow does the same--disallowing downcast once boxed into a dyn std::error::Error.

[tisonkun]

Seems not possible to override and proxy the downcast_ref of dyn std::error::Error to the inner error, so downcast will not work once Exn is wrapped into a dyn Error.

anyhow does the same--disallowing downcast once boxed into a dyn std::error::Error.

Based on our API, one can first downcast the Error to Frame and then call Frame::error to fetch the internal error and call downcast on that error instead?

[tisonkun]

Generally LGTM

We may considere how to introduce the concept of Frame to users.

Exn is the final interface and Frame wraps the inner error that users can do some advanced interaction; see examples?

[andylokandy]

Based on our API, one can first downcast the Error to Frame and then call Frame::error to fetch the internal error and call downcast on that error instead?

Yes.