Differentiate between replace and replace+delete

[matth2k]

The API has no way to differentiate between the two behaviors, making it hard to insert gates in place.

See

safety-net/tests/edits.rs

Lines 82 to 96 in 5561f90

	fn test_replace2() {
	let netlist = get_simple_example();
	let input = netlist.inputs().next().unwrap();
	let inverter = Gate::new_logical("INV".into(), vec!["I".into()], "O".into());
	let inverted = netlist
	.insert_gate_disconnected(inverter, "inst_0".into())
	.unwrap();
	// This errors, because input is not safe to delete. No replace is done.
	assert!(
	netlist
	.replace_net_uses(input.clone().unwrap(), &inverted)
	.is_err()
	);
	inverted.find_input(&"I".into()).unwrap().connect(input);
	assert_verilog_eq!(