Serializing the same hugr twice causes an error

[aborgna-q]

This test fails. (I just added the repeated validation to test_discard.

def test_discard(validate):
    @guppy
    def discard(x: int) -> None:
        return

    validate(discard)
    validate(discard)

Running pytest -v throws out this error.

validate = <function validate.<locals>.validate_impl at 0x7fddf2cf71a0>

    def test_discard(validate):
        @guppy
        def discard(x: int) -> None:
            return
    
        validate(discard)
>       validate(discard)

tests/integration/test_basic.py:36: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/integration/conftest.py:17: in validate_impl
    bs = hugr.serialize()
guppy/hugr/hugr.py:773: in serialize
    return self.to_raw().packb()
guppy/hugr/hugr.py:708: in to_raw
    self.insert_order_edges()
guppy/hugr/hugr.py:688: in insert_order_edges
    if all(
guppy/hugr/hugr.py:689: in <genexpr>
    next(self.out_edges(p), None) is None for p in n.out_ports
guppy/hugr/hugr.py:623: in out_edges
    src, tgt = self._to_edge(*e)
guppy/hugr/hugr.py:644: in _to_edge
    return src_node.out_port(key[0]), tgt_node.in_port(key[1])
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = VNode(idx=6, op=MakeTuple(parent=1, input_extensions=[], op='LeafOp', lop='MakeTuple', tys=[]), parent=BlockNode(idx=4...types=[]), meta_data={}), meta_data={}, in_port_types=[], out_port_types=[TupleType(element_types=[], preserve=False)])
offset = -1

    def in_port(self, offset: PortOffset | None) -> InPortV:
        """Returns the input port at the given offset."""
        assert offset is not None
        assert offset < self.num_in_ports
>       return InPortV(self, offset, self.in_port_types[offset])
E       IndexError: list index out of range

guppy/hugr/hugr.py:157: IndexError

This is the only tests that fails if we call hugr.serialize() twice in a row.