gob: when possible, allow sequential decoders on the same input stream.

gob: when possible, allow sequential decoders on the same input stream.
This can work only if there is no type info required to initialize the decoder,
but it's easy and gains a few percent in the basic benchmarks by avoiding
bufio when it's a bytes.Buffer - a testing-only scenario, I admit.
Add a comment about what Decode expects from the input.

R=rsc
CC=golang-dev
https://golang.org/cl/5165048