Additional cleanup with Go 1.13 as minimal version (#295)

Author: dsnet

cmp/cmpopts/equate.go

@@ -6,6 +6,7 @@
 package cmpopts
 
 import (
+"errors"
 "math"
 "reflect"
 "time"
@@ -146,3 +147,9 @@ func areConcreteErrors(x, y interface{}) bool {
 _, ok2 := y.(error)
 return ok1 && ok2
 }
+
+func compareErrors(x, y interface{}) bool {
+xe := x.(error)
+ye := y.(error)
+return errors.Is(xe, ye) || errors.Is(ye, xe)
+}

cmp/cmpopts/errors_go113.go

@@ -1,3 +1,3 @@
 module github.com/google/go-cmp
 
-go 1.11
+go 1.13