go/neo/proto: protogen: Prepare to generate code for struct heads

It is noop for 'N' encoding, but will be non-empty for MessagePack.

go/neo/proto/protogen.go

@@ -472,6 +472,9 @@ type CodeGenerator interface {
 	genArray1(path string, typ *types.Array)
 	genSlice1(path string, typ types.Type)
 
+	// generate code to process header of struct
+	genStructHead(path string, typ *types.Struct, userType types.Type)
+
 	// mem.Buf
 	genBuf(path string)
 
@@ -1236,6 +1239,14 @@ func (d *decoderN) genCustomN(path string) {
 	d.overflowCheck()
 }
 
+// ---- struct head ----
+
+// N: nothing
+func (s *sizerN)   genStructHead(path string, typ *types.Struct, userType types.Type) {}
+func (e *encoderN) genStructHead(path string, typ *types.Struct, userType types.Type) {}
+func (d *decoderN) genStructHead(path string, typ *types.Struct, userType types.Type) {}
+
+
 // top-level driver for emitting size/encode/decode code for a type
 //
 // obj is object that uses this type in source program (so in case of an error
@@ -1270,6 +1281,7 @@ func codegenType(path string, typ types.Type, obj types.Object, codegen CodeGene
 		codegen.genBasic(path, u, typ)
 
 	case *types.Struct:
+		codegen.genStructHead(path, u, typ)
 		for i := 0; i < u.NumFields(); i++ {
 			v := u.Field(i)
 			codegenType(path+"."+v.Name(), v.Type(), v, codegen)