Remove using in arrays; Remove _ non-exported struct fields

Start determining slow parts of the compiler

src/types.cpp

@@ -950,7 +950,18 @@ bool is_type_valid_for_keys(Type *t) {
 
 
 bool is_type_indexable(Type *t) {
-	return is_type_array(t) || is_type_slice(t) || is_type_vector(t) || is_type_string(t);
+	Type *bt = base_type(t);
+	switch (bt->kind) {
+	case Type_Basic:
+		return is_type_string(bt);
+	case Type_Array:
+	case Type_Slice:
+	case Type_Vector:
+	case Type_DynamicArray:
+	case Type_Map:
+		return true;
+	}
+	return false;
 }
 
 bool is_type_polymorphic_struct(Type *t) {