memory: Add free wrapper and refactor project to use it

We already use wrappers for allocation, the new `xfree` function is the
equivalent for deallocation and provides a way to fully replace the malloc
implementation used by Neovim.

src/nvim/indent.c

@@ -297,7 +297,7 @@ int set_indent(int size, int flags)
     }
     retval = true;
   } else {
-    free(newline);
+    xfree(newline);
   }
   curwin->w_cursor.col = ind_len;
   return retval;