encoding/gob: better error messages when types mismatch

encoding/gob: better error messages when types mismatch
The transmitter must encode an interface value if it is to be decoded
into an interface value, but it's a common and confusing error to
encode a concrete value and attempt to decode it into an interface,
particularly *interface{}. This CL attempts to explain things better.

Fixes #2367.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5485072