perf(map): avoid extraneous heap allocations when setting mappings

- don't immediately vim_strsave and then xfree a heap buffer.
- allow replace_termcodes to take in a buffer instead of allocating it
- grug! memory allocation bad!

src/nvim/option.c

@@ -3025,6 +3025,7 @@ ambw_end:
   } else if (varp == &p_pt) {
     // 'pastetoggle': translate key codes like in a mapping
     if (*p_pt) {
+      p = NULL;
       (void)replace_termcodes((char *)p_pt,
                               STRLEN(p_pt),
                               (char **)&p, REPTERM_FROM_PART | REPTERM_DO_LT, NULL,