This change adds new methods to Decoder.

 * Decoder.Token steps through a JSON document, returning a value for each token.
 * Decoder.Decode unmarshals the entire value at the token stream's current
   position (in addition to its existing function in a stream of JSON values)

Fixes #6050.
Fixes #6499.

Change-Id: Iff283e0e7b537221ae256392aca6529f06ebe211
Reviewed-on: https://go-review.googlesource.com/9073Reviewed-by: Russ Cox <rsc@golang.org>