rename mrb_str_buf_append to mrb_str_cat_str

The new name is better and less confusing, because:
  - `mrb_str_append` calls `mrb_str_to_str` and this function doesn't
  - `mrb_str_buf_append` _is_ `mrb_str_cat` for `mrb_value` strings

src/array.c

@@ -946,7 +946,7 @@ join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)
 
   for (i=0; i<RARRAY_LEN(ary); i++) {
     if (i > 0 && !mrb_nil_p(sep)) {
-      mrb_str_buf_append(mrb, result, sep);
+      mrb_str_cat_str(mrb, result, sep);
     }
 
     val = RARRAY_PTR(ary)[i];
@@ -958,7 +958,7 @@ join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)
 
     case MRB_TT_STRING:
     str_join:
-      mrb_str_buf_append(mrb, result, val);
+      mrb_str_cat_str(mrb, result, val);
       break;
 
     default:

src/codegen.c

@@ -1929,7 +1929,7 @@ codegen(codegen_scope *s, node *tree, int val)
       mrb_value str = mrb_str_buf_new(mrb, 4);
 
       mrb_str_cat_lit(mrb, str, "$");
-      mrb_str_buf_append(mrb, str, mrb_fixnum_to_str(mrb, fix, 10));
+      mrb_str_cat_str(mrb, str, mrb_fixnum_to_str(mrb, fix, 10));
       sym = new_sym(s, mrb_intern_str(mrb, str));
       genop(s, MKOP_ABx(OP_GETGLOBAL, cursp(), sym));
       push();

src/string.c

@@ -1254,13 +1254,6 @@ mrb_str_substr(mrb_state *mrb, mrb_value str, mrb_int beg, mrb_int len)
   return str2;
 }
 
-mrb_value
-mrb_str_buf_append(mrb_state *mrb, mrb_value str, mrb_value str2)
-{
-  mrb_str_cat(mrb, str, RSTRING_PTR(str2), RSTRING_LEN(str2));
-  return str;
-}
-
 mrb_int
 mrb_str_hash(mrb_state *mrb, mrb_value str)
 {
@@ -2468,11 +2461,17 @@ mrb_str_cat_cstr(mrb_state *mrb, mrb_value str, const char *ptr)
   return mrb_str_cat(mrb, str, ptr, strlen(ptr));
 }
 
+mrb_value
+mrb_str_cat_str(mrb_state *mrb, mrb_value str, mrb_value str2)
+{
+  return mrb_str_cat(mrb, str, RSTRING_PTR(str2), RSTRING_LEN(str2));
+}
+
 mrb_value
 mrb_str_append(mrb_state *mrb, mrb_value str, mrb_value str2)
 {
   str2 = mrb_str_to_str(mrb, str2);
-  return mrb_str_buf_append(mrb, str, str2);
+  return mrb_str_cat_str(mrb, str, str2);
 }
 
 #define CHAR_ESC_LEN 13 /* sizeof(\x{ hex of 32bit unsigned int } \0) */