zv: check the length of Type::signature for tuples/arrays/maps

[MaxVerevkin]

This assert fails:

use zvariant::Type;

fn main() {
    let illegal = T256::signature();
    assert!(illegal.len() < 256, "{} >= 256", illegal.len());
}

type T4 = (i32, i32, i32, i32);
type T16 = (T4, T4, T4, T4);
type T64 = (T16, T16, T16, T16);
type T256 = (T64, T64, T64, T64);

panicking during the signature call seems reasonable. Ideally this should be checked statically at compile time, but stable Rust isn't there yet... (although I did some research and it is possible on nightly)

Edit: a "better" const impl: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=dc8231eb9acb269988a86b7e8353dcbf

[zeenix]

panicking during the signature call seems reasonable.

As the quick solution, yes.

Edit: a "better" const impl: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=dc8231eb9acb269988a86b7e8353dcbf

const generics will make things better for sure. I have a local branch, where I've been preparing a bit for the time when Type::signature can be turned into a const.