Add second-class references to Noir

[jfecher]

Problem

Noir has mutable variables, but they are strictly local: no mutation can be observed across function boundaries. This leads to code like the following:

fn foo() {
    let mut ctx = Context::new();
    ctx.foo.bar.my_vec = ctx.foo.bar.my_vec.push(4);
}

Happy Case

If references were added, BoundedVec::push could take a mutable reference to the vector and the code could be simplified to the more natural:

fn foo() {
    let mut ctx = Context::new();
    ctx.foo.bar.my_vec.push(4);
}

However, adding first-class references comes with a host of problems. Namely, tracking the references as they are passed around everywhere, stored in structs, arrays, or returned from functions can become trouble-some. To alleviate this, we should only add second-class references to Noir. Second-class references will not be allowed to be stored within structs or arrays, and can only be used essentially as a parameter-passing mode. This way our existing analysis for references will suffice while still gaining us some increased ergonomics.

Alternatives Considered

No response

Additional Context

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Would you like to submit a PR for this Issue?

No

Support Needs

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