This CL allows JSON-encoding & -decoding maps whose keys are types that
implement encoding.TextMarshaler / TextUnmarshaler.

During encode, the map keys are marshaled upfront so that they can be
sorted.

Fixes #12146

Change-Id: I43809750a7ad82a3603662f095c7baf75fd172da
Reviewed-on: https://go-review.googlesource.com/20356
Run-TryBot: Caleb Spare <cespare@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>