Add "[must-use]" attribute into collect vec

[Pzixel]

This might lead to the problems as following:

    pub fn unique_id(&self) -> impl Hash + PartialEq + Eq {
        self.items.iter().map(|x| x.id_string.clone()).collect::<Vec<_>>();
    }

This code has a typo and rust warns against it:

unused return value of collect that must be used
if you really need to exhaust the iterator, consider .for_each(drop) instead
#[warn(unused_must_use)] on by defaultrustcClick for full compiler diagnostic
mod.rs(964, 9): use let _ = ... to ignore the resulting value: let _ =

If you replace it with following:

    pub fn unique_id(&self) -> impl Hash + PartialEq + Eq {
        self.items.iter().map(|x| x.id_string.clone()).collect_vec();
    }

No warning will be generated, as no must_use attribute exist on the function.

This PR adds it, while small, it can lead to huge problems, since () implements a lot of traits and impl Trait are used very often with iterators, so you can erroneously use one instead of another.

P.S. I also noticed that try_collect only works with #[cfg(feature = "use_alloc")], but try_collect doesn't. I'm not sure if this is intended.