fix: fix an ICE happening when we call a closure result from if/else

[alehander92]

Description

Fix an ICE resulting of If nodes having fn type when a closure is returned, while monomorphization rewrites mean, the actual type should be a Tuple: happening when we call a closure preserved in a variable, resulting from such an if.

fixes #2154

Problem*

an example found by @jfecher : (fixing the first example only!)

#1959 (comment)

fn main() {
    let x: u32 = 32;

    let f = if x > 2 {
        || x
    } else {
        || x + 2342
    };

    dep::std::println(f());
}

was failing: this should fix it

Summary*

changed convert_type in Monomorphizer to return a Tuple for closures, suggester by jfetcher in #1959 (comment)

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.

[alehander92]

the CI wasm build failure seems mirror-related, not related to the code from what i see

[jfecher]

Thanks for the shorter PR, this was much quicker to review

[alehander92]

@jfecher thanks, tried to address the reviews, opened an issue, and rebased