gob: enable the GobEncoder and GobDecoder interfaces.

gob: enable the GobEncoder and GobDecoder interfaces.
These allow data items to control their own representation.

For now, the implementation requires that the value passed
to Encode and Decode must be exactly the type of the
methods' receiver; it cannot be, for instance, T if the receiver
is of type *T.  This will be fixed in a later CL.

R=rsc
CC=golang-dev
https://golang.org/cl/4235051