The order package replaces the mapsort and fieldsort packages.
It presents a common API for ordered iteration over message fields
and map fields.
It has a number of pre-defined orderings.
Change-Id: Ie6cd423da30b4757864c352cb04454f21fe07ee2
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/239837
Reviewed-by: Herbie Ong <herbie@google.com>
