Allow message implementations to provide optimized versions of standard
operations. Generated messages now include a ProtoReflectMethods method,
returning a protoiface.Methods struct containing pointers to assorted
optional functions.
The Methods struct also includes a Flags field indicating support for
optional features such as deterministic marshaling.
Implementation of the fast paths (and tests) will come in later CLs.
Change-Id: Idd1beed0ecf43ec5e5e7b8da2ee1e08d3ce32213
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/170340
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
