Rewrite constant_fold_pass using dataflow framework

[acl-cqc]

Remnants of #1157 after the "framework" of #1476 is pulled out, plus transformation roughly paralleling the old constant-folding code + dead-code-elimination. (Eliminating cases from conditionals/tail-loops and transforming into DFGs, left for another PR.)

All the existing constant-folding tests pass! 😀 🚀, plus at least one that didn't just fail-to-optimize, but panicked - or, in release mode, potentially produced an invalid hugr - see comment.

Hence, feels like it's time this had some eyes, also as guidance for reviewers of #1476.

closes #1322

[doug-q]

Suggested change

	let (src, outp) = hugr.single_linked_output(n, ip).unwrap();
	let (src, outp) = hugr.single_linked_output(n, ip)?;

I had to make this change locally after merging main, to fix failing tests. I believe inputs are often disconnected when working with various subgraph views.

[doug-q]

Suggested change

	let sig = op.signature();
	let known_ins = sig
	.input_types()
	.iter()
	.enumerate()
	.zip(ins.iter())
	.filter_map(|((i, ty), pv)| {
	Some((IncomingPort::from(i), pv.clone().try_into_value(ty).ok()?))
	})
	.collect::<Vec<_>>();
	for (p, v) in op.constant_fold(&known_ins).unwrap_or_default() {
	outs[p.index()] =
	partial_from_const(self, ConstLocation::Field(p.index(), &node.into()), &v);
	}
	}
	// If any inputs are bottom this node is currently unreachable.
	// We should not propagate anything to our outputs here.
	// TODO perhaps this should be done by the caller and we should have a
	// precondition here that no ins are Bottom; this if statement
	// would become a debug_assert!.
	if ins.iter().any(|v| *v == PartialValue::Bottom) {
	return;
	}
	let sig = op.signature();
	let known_ins = sig
	.input_types()
	.iter()
	.enumerate()
	.zip(ins.iter())
	.filter_map(|((i, ty), pv)| {
	Some((IncomingPort::from(i), pv.clone().try_into_value(ty).ok()?))
	})
	.collect::<Vec<_>>();
	// Collect the results of the fold. Any output wires that are not
	// returned by `constant_fold` are set to Top.
	let fold_outs: HashMap<OutgoingPort, Value> = op
	.constant_fold(&known_ins)
	.into_iter()
	.flat_map(Vec::into_iter)
	.collect();
	for (p, o) in outs.iter_mut().enumerate() {
	o.join_mut(
	fold_outs
	.get(&OutgoingPort::from(p))
	.map_or(PartialValue::Top, |v| {
	partial_from_const(self, node, &mut vec![p.index()], v)
	}),
	);
	}