fix: Fix an ICE when reassigning a mutable lambda variable to one with a different environment type

[alexvitkov]

Description

Problem

This code type-checks properly, but an assertion fails during SSA generation.
The reassignment is invalid since the two lambdas have different environment types, and should be caught during type-checking.

fn main() {
    let a: u32 = 1;
    let b: u32 = 1;

    let mut f = || a;
    f = || a + b;

    f();
}

Resolves #2139

Summary

This adds an additional check to assignments, to cover this case and throw an error if an invalid assignment happens.

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.

[jfecher]

We shouldn't need a check specifically for closures in assignments which don't otherwise have any closure-specific logic. If we were to go forward with this approach, we'd need a closure-specific check everywhere two closures may be unified (e.g. if statements, storing them in structs, arrays, etc). We should instead make it so that two closures with different environment types do not unify or subtype with each other.

[alexvitkov]

Fair enough. fixed in dfc0ba0