shared/log15/stack/stack.go
func Callers() Trace {
	pcs := poolBuf()
	pcs = pcs[:cap(pcs)]
	n := runtime.Callers(2, pcs)
	cs := make([]Call, n)
	for i, pc := range pcs[:n] {
		cs[i] = Call(pc)
	}
	putPoolBuf(pcs)
	return cs
}
func Callers() Trace {
	pcs := poolBuf()
	pcs = pcs[:cap(pcs)]
	n := runtime.Callers(2, pcs)
	cs := make([]Call, n)
	for i, pc := range pcs[:n] {
		cs[i] = Call(pc)
	}
	putPoolBuf(pcs)
	return cs
}
// Callers returns a Trace for the current goroutine with element 0
// identifying the calling function.