Always pass return_value_ptr to internal functions

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.
Make use of direct returns in some places