Turn value_fetch_lazy into a method

This changes value_fetch_lazy to be a method of value.  A few helper
functions are converted as well, to avoid problems in later patches
when the data members are all made private.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/python/py-value.c

@@ -1241,7 +1241,7 @@ valpy_fetch_lazy (PyObject *self, PyObject *args)
   try
     {
       if (value->lazy ())
-	value_fetch_lazy (value);
+	value->fetch_lazy ();
     }
   catch (const gdb_exception &except)
     {