cmd/protoc-gen-go: change some arrays to slices to save bytes

Using arrays in the generated reflection information adds unnecessary
eq and hash functions being added to the package. Change to slices
to reduce bloat.

Change-Id: I1a4f6d59021644d93dd6c24679b9233141e89a75
Reviewed-on: https://go-review.googlesource.com/c/164640
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 7144ae8..5533185 100644
--- a/internal/testprotos/test/test_public.pb.go
+++ b/internal/testprotos/test/test_public.pb.go
@@ -74,22 +74,22 @@
 
 var File_test_test_public_proto protoreflect.FileDescriptor
 
-var xxx_File_test_test_public_proto_messageTypes [1]protoimpl.MessageType
+var xxx_File_test_test_public_proto_messageTypes = make([]protoimpl.MessageType, 1)
 var xxx_File_test_test_public_proto_goTypes = []interface{}{
 	(*PublicImportMessage)(nil), // 0: goproto.proto.test.PublicImportMessage
 }
 var xxx_File_test_test_public_proto_depIdxs = []int32{}
 
 func init() {
-	var messageTypes [1]protoreflect.MessageType
+	messageTypes := make([]protoreflect.MessageType, 1)
 	File_test_test_public_proto = protoimpl.FileBuilder{
 		RawDescriptor:      xxx_File_test_test_public_proto_rawdesc,
 		GoTypes:            xxx_File_test_test_public_proto_goTypes,
 		DependencyIndexes:  xxx_File_test_test_public_proto_depIdxs,
-		MessageOutputTypes: messageTypes[:],
+		MessageOutputTypes: messageTypes,
 	}.Init()
 	messageGoTypes := xxx_File_test_test_public_proto_goTypes[0:][:1]
-	for i, mt := range messageTypes[:] {
+	for i, mt := range messageTypes {
 		xxx_File_test_test_public_proto_messageTypes[i].GoType = reflect.TypeOf(messageGoTypes[i])
 		xxx_File_test_test_public_proto_messageTypes[i].PBType = mt
 	}