encoding/gob: avoid allocating string for map key

On linux/386 compared to tip:

	name                     old time/op  new time/op  delta
	DecodeInterfaceSlice-40  1.23ms ± 1%  1.17ms ± 1%  -4.93%  (p=0.000 n=9+10)

Recovers about half the performance regression from Go 1.6 on 386.

For #16117.

Change-Id: Ie8676d92a4da3e27ff21b91a98b3e13d16730ba1
Reviewed-on: https://go-review.googlesource.com/24468Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/gob/decode.go

@@ -645,10 +645,10 @@ func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, valu
 		errorf("invalid type name length %d: exceeds input size", nr)
 	}
 	n := int(nr)
-	name := string(state.b.Bytes()[:n])
+	name := state.b.Bytes()[:n]
 	state.b.Drop(n)
 	// Allocate the destination interface value.
-	if name == "" {
+	if len(name) == 0 {
 		// Copy the nil interface value to the target.
 		value.Set(reflect.Zero(value.Type()))
 		return
@@ -658,7 +658,7 @@ func (dec *Decoder) decodeInterface(ityp reflect.Type, state *decoderState, valu
 	}
 	// The concrete type must be registered.
 	registerLock.RLock()
-	typ, ok := nameToConcreteType[name]
+	typ, ok := nameToConcreteType[string(name)]
 	registerLock.RUnlock()
 	if !ok {
 		errorf("name not registered for interface: %q", name)