add rh_values() method and test for it

`rh_values()` is a shortcut for 

    my $length = $glade->rh_length();
    my @return_value = map { $glade->rh_value($_) } 0 .. $length - 1;

as used in `sl_panda.t`. 

Also, if I got Deyan's idea right, it could be useful for [`rule_closure()`](https://github.com/jeffreykegler/Marpa--R2/issues/78):

    my $rule_id     = $glade->rule_id();
    my $closure    = $grammar->rule_closure($rule_id);

    my @semantic_values = $closure->( $glade->rh_values() );

which I plan testing in `sl_panda1.t`.