Compression: automatically propagate constants and remove unused nodes

[VincentDerk]

Re-enables the propagation of constants (true or false), and the removal of unused nodes (those not connected to a root node).

• The propagation of constants happens automatically while adding nodes to the circuit.
• The removal happens when calling circuit.tensorize(), or when manually calling circuit.remove_unused_nodes().
  Users should be aware that Python references to unused nodes become invalid after calling .tensorize().

This PR adds additional tests for the propagation and elimination. I confirm all tests pass, also those in DeepLog.

This PR additionally exposes through the Python interface: .nb_root_nodes(), .remove_unused_nodes(), and klay.utils.circuit_to_dot(circuit, dot_filepath). The latter is more for debugging purposes.

Worst case in the future, we can again quickly disable this like before

Node* Circuit::add_node_level_compressed(Node* node) {
    // return add_node_level(node); // <--- uncomment to disable
    ...

std::pair<Arrays, Arrays> Circuit::tensorize() {
    remove_unused_nodes();  // <--- comment to disable
    add_root_layer();
    ....

[jjcmoon]

lgtm!