feat: subgraphs within graph

[alexander-camuto]

The Scan op in tract defines subgraphs within a graph. This is how, in particular, recursive models like RNNs and LSTMs are represented. Here we introduce the ability for models to be nodes of a larger meta-model.

This is done by defining:

/// Enables model as subnode of other models
#[derive(Clone, Debug)]
pub enum NodeType<F: PrimeField + TensorType + PartialOrd> {
    /// A node in the model
    Node(Node<F>),
    /// A submodel
    SubGraph {
        /// The subgraph
        graph: Model<F>,
        /// The subgraph's inputs
        inputs: Vec<usize>,
        /// the subgraph's idx within the parent graph
        idx: usize,
    },
}

The Nodes of a models are then:

/// Representation of execution graph
pub type NodeGraph<F> = BTreeMap<usize, NodeType<F>>;

This enables us to define 99% of LSTM / RNN layers. Resolving most of #164

Remaining OPs to implement are concat and slice :)