encoding/gob: avoid allocating string for map key

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/24468
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>