fix: Prevent mutating immutable bindings to mutable reference types

[jfecher]

Description

Problem*

Finally Resolves #2911. There were several issues hidden there.

Summary*

In the simplified source code from the original bug:

#[test]
fn mutable_return_error() {
    let slice : &mut [Field] = &mut [];
    slice = &mut (*slice).push_back(1);
    assert((*slice)[0] == 1);
}

It turns out the panic was actually correct - it should have been unreachable. It stemmed from the mutation on slice when slice was not declared as mutable. Importantly, you can mutate mutable references through dereferencing them, but you should not have been able to swap out the reference itself.

I've fixed the mutability check to allow *slice = slice.push_back(1) (as before), but now disallow slice = &mut ... unless slice was also marked mutable.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[kevaundray]

Ah let's add a regression test to this

[jfecher]

It looks like there was a bad merge since the diff of this PR got messed up. I'm just going to open another PR.