Make stackTracer interface public

[hnakamur]

I read https://godoc.org/github.com/pkg/errors#hdr-Retrieving_the_stack_trace_of_an_error_or_wrapper
and learn you need to the stackTracer interface yourself.

I understand the intension is exposing APIs as little as possible to enhance the future API compatibility.
However, if everyone must defines the stackTracer interface to retrieve stack traces, it is actually an implicit interface. I think it is not good.

So how about make the stackTracer interface public and adding StackTracer interface?

[davecheney]

I understand this request but I'm sorry I do not plan to add this interface to this package. I strongly believe that interfaces should be defined by their caller, not the package which implements them.

Thank you for your understanding.

[hnakamur]

OK, I understand.
Thanks for your explanation.