Support for obtaining a stack trace without requiring this package

[mitar]

I made a spiritual successor of github.com/pkg/errors some time ago and in it I want to know if an error already contains a stack trace. The goal is that if a large codebase uses different packages for errors, they should work together and not hide existing recorded information, like a stack trace.

From what I am seeing in this package is that it has the same issue as github.com/pkg/errors: the return value of StackTrace() is a custom type defined in this package. So it is not possible to detect the error having this method without importing this package.

I would suggest that instead, the following interface is implemented:

type stackTracer interface {
        StackTrace() []uintptr
}

Then, it is easy to obtain a stack trace, use it, format it, and one does not have to depend on a library which created the error.

To prevent a breaking change, I have seen also the following interface being used in this package:

type goErrorsStackTracer interface {
	Callers() []uintptr
}

BTW, you are also using runtime.FuncForPC which is deprecated. And calling it runtime.FuncForPC at the time of making a stack trace is quite a performance hit. It is much better to just record a stack trace (into []uintptr) when making an error and then resolve addresses with runtime.FuncForPC when the stack is formatted (if ever).