ParseXS: refactor: give $build_subclass parent arg

For aesthetic reasons, give the $build_subclass sub an extra first arg
which must be the string 'parent'. Then change invocations from:

    BEGIN { $build_subclass->('Foo', # parent
        'field1', # ...
        ...
    }

to
    BEGIN { $build_subclass->(parent => 'Foo',
        'field1', # ...
        ...
    }

Author: iabyn