encoding/json: add Decoder.Buffered accessor to get overread data

Otherwise it's impossible to know how much data from the
json.Decoder's underlying Reader was actually consumed.

The old fix from golang.org/issue/1955 just added docs. This
provides an actual mechanism.

Update issue 1955

R=golang-dev, adg, rsc
CC=golang-dev
https://codereview.appspot.com/7181053