feat(mpz-circuits-generic): implement generic circuit struct

[brech1]

Description

This PR aims to introduce a new generic circuit struct:

pub struct Circuit<T> {
    input_count: usize,
    output_count: usize,
    gates: Vec<T>,
}

The Circuit is a collection of gates. Since for the execution of these gates is important that these form a directed acyclic graph (DAG), the correct way of building this circuit is using the CircuitBuilder

pub struct CircuitBuilder<T> {
    current_node: Node,
    inputs: Vec<Node>,
    outputs: Vec<Node>,
    gates: Vec<T>,
    stack_size: usize,
}

The CircuitBuilder API ensures that the gates are ordered topologically, this means in the order they should be executed taking dependencies into account

Here is our only constrain for the gates, they have to implement the Component trait, this means that they should be able to return an iterator over the nodes.

pub trait Component {
    fn get_inputs(&self) -> impl Iterator<Item = &Node>;
    fn get_outputs(&self) -> impl Iterator<Item = &Node>;
}

The Node struct:

pub struct Node(pub(crate) u32);

Example

let mut builder = CircuitBuilder::<Gate>::new();

let (in_0, in_1) = (builder.add_input(), builder.add_input());

let &Gate { output, .. } = builder
    .add_gate(|next| Gate {
        inputs: vec![in_0, in_1],
        output: next.next(),
    })
    .unwrap();
    
let &Gate { output, .. } = builder
    .add_gate(|next| Gate {
        inputs: vec![in_0, output],
        output: next.next(),
    })
    .unwrap();

builder.add_output(output);