proto, runtime/protoiface: add support for fast-path marshaling

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>
diff --git a/internal/testprotos/test/test_public.pb.go b/internal/testprotos/test/test_public.pb.go
index fde1bd6..1c76763 100644
--- a/internal/testprotos/test/test_public.pb.go
+++ b/internal/testprotos/test/test_public.pb.go
@@ -6,6 +6,7 @@
 import (
 	protoreflect "github.com/golang/protobuf/v2/reflect/protoreflect"
 	protoregistry "github.com/golang/protobuf/v2/reflect/protoregistry"
+	protoiface "github.com/golang/protobuf/v2/runtime/protoiface"
 	protoimpl "github.com/golang/protobuf/v2/runtime/protoimpl"
 	sync "sync"
 )
@@ -32,6 +33,10 @@
 	return xxx_File_test_test_public_proto_messageTypes[0].MessageOf(x)
 }
 
+func (m *PublicImportMessage) XXX_Methods() *protoiface.Methods {
+	return xxx_File_test_test_public_proto_messageTypes[0].Methods()
+}
+
 // Deprecated: Use PublicImportMessage.ProtoReflect.Type instead.
 func (*PublicImportMessage) Descriptor() ([]byte, []int) {
 	return xxx_File_test_test_public_proto_rawDescGZIP(), []int{0}