errors: deprecate functions that are now in standard library

Change-Id: I8dc2a0d3923fdd55bcc205eea99142b1a32d569d
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/399607
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

fmt.go

diff --git a/fmt.go b/fmt.go
index 829862d..6df1866 100644
--- a/fmt.go
+++ b/fmt.go
@@ -33,6 +33,8 @@
 // It is invalid to include more than one %w verb or to supply it with an
 // operand that does not implement the error interface. The %w verb is otherwise
 // a synonym for %v.
+//
+// Deprecated: As of Go 1.13, use fmt.Errorf instead.
 func Errorf(format string, a ...interface{}) error {
 	format = formatPlusW(format)
 	// Support a ": %[wsv]" suffix, which works well with xerrors.Formatter.

wrap.go

diff --git a/wrap.go b/wrap.go
index 9a3b510..9842758 100644
--- a/wrap.go
+++ b/wrap.go
@@ -35,6 +35,8 @@
 
 // Unwrap returns the result of calling the Unwrap method on err, if err implements
 // Unwrap. Otherwise, Unwrap returns nil.
+//
+// Deprecated: As of Go 1.13, use errors.Unwrap instead.
 func Unwrap(err error) error {
 	u, ok := err.(Wrapper)
 	if !ok {
@@ -47,6 +49,8 @@
 //
 // An error is considered to match a target if it is equal to that target or if
 // it implements a method Is(error) bool such that Is(target) returns true.
+//
+// Deprecated: As of Go 1.13, use errors.Is instead.
 func Is(err, target error) bool {
 	if target == nil {
 		return err == target
@@ -77,6 +81,8 @@
 //
 // The As method should set the target to its value and return true if err
 // matches the type to which target points.
+//
+// Deprecated: As of Go 1.13, use errors.As instead.
 func As(err error, target interface{}) bool {
 	if target == nil {
 		panic("errors: target cannot be nil")