- Improved memory usage

- Improved memory usage
  . zend_function.pass_rest_by_reference is replaced by
    ZEND_ACC_PASS_REST_BY_REFERENCE in zend_function.fn_flags
  . zend_function.return_reference is replaced by ZEND_ACC_RETURN_REFERENCE
    in zend_function.fn_flags
  . zend_arg_info.required_num_args removed. it was needed only for internal
    functions. Now the first arg_info for internal function (which has special
    meaning) is represented by zend_internal_function_info structure.
  . zend_op_array.size, size_var, size_literal, current_brk_cont,
    backpatch_count moved into CG(context), because they are used only during
    compilation.
  . zend_op_array.start_op is moved into EG(start_op), because it's used
    only for 'interactive' execution of single top-level op-array.
  . zend_op_array.done_pass_two is replaced by ZEND_ACC_DONE_PASS_TWO in
    zend_op_array.fn_flags.
  . op_array.vars array is trimmed (reallocated) during pass_two.
  . zend_class_entry.constants_updated is replaced by
     ZEND_ACC_CONSTANTS_UPDATED in zend_class_entry.ce_flags
  . the size of zend_class_entry is reduced by sharing the same memory space
    by different information for internal and user classes.
    See zend_class_inttry.info union.