use mrb_str_cat_str() instead of mrb_str_append()

If the argument is always a string, then mrb_str_cat_str() can be
directly called instead of indirectly by mrb_str_append().

mrb_any_to_s(), mrb_obj_as_string(), mrb_inspect() always return a
string.

src/class.c

@@ -1608,10 +1608,10 @@ mrb_mod_to_s(mrb_state *mrb, mrb_value klass)
       case MRB_TT_CLASS:
       case MRB_TT_MODULE:
       case MRB_TT_SCLASS:
-        mrb_str_append(mrb, str, mrb_inspect(mrb, v));
+        mrb_str_cat_str(mrb, str, mrb_inspect(mrb, v));
         break;
       default:
-        mrb_str_append(mrb, str, mrb_any_to_s(mrb, v));
+        mrb_str_cat_str(mrb, str, mrb_any_to_s(mrb, v));
         break;
     }
     return mrb_str_cat_lit(mrb, str, ">");

src/error.c

@@ -152,7 +152,7 @@ exc_inspect(mrb_state *mrb, mrb_value exc)
     mrb_str_append(mrb, str, line);
     mrb_str_cat_lit(mrb, str, ": ");
     if (append_mesg) {
-      mrb_str_append(mrb, str, mesg);
+      mrb_str_cat_str(mrb, str, mesg);
       mrb_str_cat_lit(mrb, str, " (");
     }
     mrb_str_cat_cstr(mrb, str, mrb_obj_classname(mrb, exc));
@@ -165,7 +165,7 @@ exc_inspect(mrb_state *mrb, mrb_value exc)
     str = mrb_str_new_cstr(mrb, cname);
     mrb_str_cat_lit(mrb, str, ": ");
     if (append_mesg) {
-      mrb_str_append(mrb, str, mesg);
+      mrb_str_cat_str(mrb, str, mesg);
     }
     else {
       mrb_str_cat_cstr(mrb, str, cname);

src/range.c

@@ -290,7 +290,7 @@ range_to_s(mrb_state *mrb, mrb_value range)
   str2 = mrb_obj_as_string(mrb, r->edges->end);
   str  = mrb_str_dup(mrb, str);
   mrb_str_cat(mrb, str, "...", r->excl ? 3 : 2);
-  mrb_str_append(mrb, str, str2);
+  mrb_str_cat_str(mrb, str, str2);
 
   return str;
 }
@@ -315,7 +315,7 @@ range_inspect(mrb_state *mrb, mrb_value range)
   str2 = mrb_inspect(mrb, r->edges->end);
   str  = mrb_str_dup(mrb, str);
   mrb_str_cat(mrb, str, "...", r->excl ? 3 : 2);
-  mrb_str_append(mrb, str, str2);
+  mrb_str_cat_str(mrb, str, str2);
 
   return str;
 }

src/variable.c

@@ -609,7 +609,7 @@ inspect_i(mrb_state *mrb, mrb_sym sym, mrb_value v, void *p)
   else {
     ins = mrb_inspect(mrb, v);
   }
-  mrb_str_append(mrb, str, ins);
+  mrb_str_cat_str(mrb, str, ins);
   return 0;
 }