Remove Clone requirement for slots in Vec

[Senzaki]

This PR removes the requirement for S: Clone for when rendering a list of slots S.

It is currently required because the view! macro is building an array of S, then calling to_vec() (which obviously requires S: Clone). This can be a problem when the slot contains ChildrenFn (i.e. Box<dyn Fn(Scope) -> Fragment), as this type does not implement Clone.

This PR removes this requirement by using the vec! macro directly instead.

Take this code for example:

use leptos::{component, mount_to_body, slot, view, ChildrenFn, CollectView, IntoView, Scope};

#[slot]
pub struct MenuItems {
    pub menu_item: Vec<MenuItem>,
}

#[slot]
pub struct MenuItem {
    pub children: ChildrenFn,
}

#[component]
pub fn Menu(cx: Scope, menu_items: MenuItems) -> impl IntoView {
    view! {
        cx,
        <ul>
            {
                menu_items
                    .menu_item
                    .into_iter()
                    .map(|item| view! {
                        cx,
                        <li>{ (item.children)(cx) }</li>
                    })
                    .collect_view(cx)
            }
        </ul>
    }
}

fn main() {
    mount_to_body(|cx| {
        view! {
            cx,
            <Menu>
                <MenuItems slot>
                    <MenuItem slot>"hello"</MenuItem>
                    <MenuItem slot>"world"</MenuItem>
                </MenuItems>
            </Menu>
        }
    });
}

With the current version of leptos, it fails to compile with the following message:

error[E0277]: the trait bound `MenuItem: Clone` is not satisfied
note: required by a bound in `std::slice::<impl [T]>::to_vec`

With this PR, it build properly.

[Senzaki]

The CI fail does not seem to be a regression because of this commit, it might be an ICE because of a new version of rust nightly, as it also occurs on the main branch.

Running the following commands fails on both main and on my branch slot-vec-without-clone:

cd leptos_macro/example
cargo +nightly doc

It fails in the bumpalo dependency, which has not published a new version since May (so it mustn't be this crate's fault either?)

[Senzaki]

I have also confirmed it works with a previous version of rust nightly (cargo +nightly-2023-08-15 doc), so it is indeed a rust nightly ICE. Must be this issue.

[gbj]

Thanks for the PR! I'll try to review it this week. Agreed that this is an ICE on most recent nightly, affecting several branches here. I can run CI locally if need be.

[gbj]

This looks good to me. Thanks very much.