go/neo/proto: protogen: Clarify criteria to detect []byte

We previously used criteria that sizeof(typ.Elem()) == 1, but it would
also trigger on e.g. `struct { x byte }` which we would not want to
consider as byte array.

-> Clarify the test to explicitly test for typ.Elem() == byte.
No change in generated code.

go/neo/proto/protogen.go

@@ -449,13 +449,6 @@ notfixed:
 	return 0, false
 }
 
-// does a type have fixed wire size == 1 ?
-func typeSizeFixed1(typ types.Type) bool {
-	wireSize, _ := typeSizeFixed(typ)
-	return wireSize == 1
-}
-
-
 // interface of a codegenerator  (for sizer/coder/decoder)
 type CodeGenerator interface {
 	// tell codegen it should generate code for which type & receiver name
@@ -1222,7 +1215,7 @@ func codegenType(path string, typ types.Type, obj types.Object, codegen CodeGene
 
 	case *types.Array:
 		// [...]byte or [...]uint8 - just straight copy
-		if typeSizeFixed1(u.Elem()) {
+		if isByte(u.Elem()) {
 			codegen.genArray1(path, u)
 		} else {
 			var i int64
@@ -1232,7 +1225,7 @@ func codegenType(path string, typ types.Type, obj types.Object, codegen CodeGene
 		}
 
 	case *types.Slice:
-		if typeSizeFixed1(u.Elem()) {
+		if isByte(u.Elem()) {
 			codegen.genSlice1(path, u)
 		} else {
 			codegen.genSlice(path, u, obj)
@@ -1258,3 +1251,9 @@ func generateCodecCode(typespec *ast.TypeSpec, codegen CodeGenerator) string {
 
 	return codegen.generatedCode()
 }
+
+// isByte returns whether typ represents byte.
+func isByte(typ types.Type) bool {
+	t, ok := typ.(*types.Basic)
+	return ok && t.Kind() == types.Byte
+}