gob: remove a few more allocations.

gob: remove a few more allocations.
- use enc.err and dec.err instead of return values in deferred error catcher
- replace io.WriteString with buffer.WriteString

now at:
mallocs per encode of type Bench: 7
mallocs per decode of type Bench: 8

R=rsc
CC=golang-dev
https://golang.org/cl/4277057