Add TraceCaller interface for extensibility for #104

[StevenACoffman]

Fixes #104

This PR extends errtrace functionality to accommodate custom errors that honor the new TraceCaller interface contract.

For instance, a custom error can have arbitrary fields as described in #36 and as long as it also contains a TraceCall() receiver function, it can be included in the errtrace.

type myTraceableError struct {
	// Fields are key-value pairs
	attrs    []slog.Attr

	// Wrapped Error
	err error

	// PC is the program counter for the location in this frame.
	// For a frame that calls another frame, this will be the
	// program counter of a call instruction. Because of inlining,
	// multiple frames may have the same PC value, but different
	// symbolic information.
	pc  uintptr
}

func (e *myTraceableError) TraceCall() uintptr {
	return e.pc
}

[abhinav]

Thanks, @StevenACoffman. This generally looks good, but I'd like to request a couple changes:

• Rename TraceCall to TracePC. I like having "Trace" in there since it's specifically "this error is contributing this PC/frame to the trace." Naming it "TracePC" leaves us room to allow contributing a frame directly in the future should we ever decide to use non-PC inputs.
• Unexport the TraceCaller interface per this comment. There aren't many valid use cases for people to reference the interface type directly; they'll only ever implement it. The FormatString/Format functions can mention that if an error implements the "TracePC() uintptr" method, it'll contribute to the trace.
• Unexport UnwrapOnce. This seems like an implementation detail that users of errtrace don't need to use directly.

[StevenACoffman]

Sounds good! All done.

[prashantv]

Thanks for the contribution @StevenACoffman

[abhinav]

LGTM minus one comment.
Thanks, @StevenACoffman.

CC @prashantv

[StevenACoffman]

I agree that inner is a better name. wrapErr was a silly play on wrapper, but I still get to giggle when any error name's last syllable can be pronounced err. 😄

[abhinav]

unclear what's going on with coverage. Patch coverage is 100%.
Safe to ignore.

@prashantv will merge later today; will give you a chance to look over first.

[prashantv]

LGTM