gob: protect against pure recursive types.

gob: protect against pure recursive types.

There are further changes required for things like
recursive map types.  Recursive struct types work
but the mechanism needs generalization.  The
case handled in this CL is pathological since it
cannot be represented at all by gob, so it should
be handled separately. (Prior to this CL, encode
would recur forever.)

R=rsc
CC=golang-dev
https://golang.org/cl/4206041