Always provide retval ptr

Previous some places passed `return_value_ptr` only if the function returned by reference. Now `return_value_ptr` is always set, even for functions returning by-value.

This allows you to return zvals without copying their contents. For this purpose two new macros 
`RETVAL_ZVAL_FAST` and `RETURN_ZVAL_FAST` are added:

    RETVAL_ZVAL_FAST(zv); /* Analog to RETVAL_ZVAL(zv, 1, 0) */
    RETURN_ZVAL_FAST(zv); /* Analog to RETURN_ZVAL(zv, 1, 0) */

These macros behave similarly to the non-FAST versions with copy=1 and dtor=0, with the difference that the FAST versions will try return the zval without copying by utilizing `return_value_ptr`.