[Plonky3 Codegen] Expression de-duplication in generated constraints

When compiling the MidenVM constraints currently done towards the Plonky3 backend, the generated constraints represent ~70 individual constraints.
However, the RPO constraints are written in a way that makes it hard to load (one constraint being represented with 1 line of 80k+ caracters).

We should maybe better optimize some expressions. For instance, we could improve the Common SubExpression Elimination: currently, it deduplicates nodes in the algebraic graph, but the codegen duplicates the generated Rust code back.

Using Rust `let` statements when a node is duplicated (maybe with some stricter conditions such as "this node should be referenced more than 10 times in the graph to have a dedicated let statement") would certainly improve readability.

The main issue I think would be that the variable names would probably not be made easily readable.

Look at the Plonky3 codegen in the MidenVM constraints generation, focusing on the algebraic graph and Common SubExpression Elimination (CSE). The goal is to deduplicate expressions in generated Rust code by introducing `let` statements for frequently referenced nodes. Start by examining the codegen module to understand how constraints are currently emitted and where CSE is applied. Check for conditions to decide when to create a `let` statement (e.g., node referenced more than 10 times). The outcome should improve readability by reducing duplication in the generated Rust code.

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