Change the semantics of exprs/for loops allocations strategy

[jkelleyrtp]

This commit adjusts how exprs and for loops are handled within rsx. This is a breaking change in terms of codegen, but has slight semantic changes as well.

Now, when exprs/for loops are allocated, they are given a temporary. The temporary is elided to the <'a> lifetime of the bump, to satisfy the borrow checker. This fixes issues with signals where exprs/for loops mapping vecs out of RefCells would be caught up without a temporary lifetime.

Specifically, before, this component could not compile:

fn app(cx: Scope) -> Element {
    let todos = use_signal(cx, || vec!["hi"]);

    render! {
        ul {
            for todo in todos.read().iter() {
                li { "{todo}" }
            }

            todos.read().iter().map(|todo| rsx! {
                li { "{todo}" }
            })
        }
    }
}

The error we were getting is:

error[E0597]: `todos` does not live long enough
  --> src/main.rs:13:25
   |
9  |     let todos = use_signal(cx, || vec!["hi"]);
   |         ----- binding `todos` declared here
...
13 |             for todo in todos.read().iter() {
   |                         ^^^^^^^^^^^^
   |                         |
   |                         borrowed value does not live long enough
   |                         a temporary with access to the borrow is created here ...
...
22 | }
   | -
   | |
   | `todos` dropped here while still borrowed
   | ... and the borrow might be used here, when that temporary is dropped and runs the destructor for type `std::cell::Ref<'_, std::vec::Vec<&str>>`
   |
   = note: the temporary is part of an expression at the end of a block;
           consider forcing this temporary to be dropped sooner, before the block's local variables are dropped
help: for example, you could save the expression's value in a new local variable `x` and then make `x` be the expression at the end of the block
   |
11 ~     let x = render! {
12 |         ul {
 ...
20 |         }
21 ~     }; x
   |

For more information about this error, try `rustc --explain E0597`.
error: could not compile `demo-signals-todo` (bin "demo-signals-todo") due to previous error

This is fixed now, fixing a major papercut of signals and the work being done in fermi to allow truly global state.

This works because we now do a codegen from

make_node(#expr)

to

{
    let node = #expr.make_node(cx);
    node
}

[ealmloff]

This makes rsx slightly more forgiving and everything gets converted to Vnodes when the component is diffed anyway so it shouldn't have a large performance impact.

A very similar issue can happen with let statements in unterminated if blocks that could be worth addressing in this PR as well:

fn app(cx: Scope) -> Element {
    let todos = use_ref(cx, || vec!["hi"]);

    render! {
        ul {
            if let &[first] = todos.read().as_slice() {
                rsx! { li { "single element: {first}" } }
            }
        }
    }
}

[jkelleyrtp]

It technically worked before, apparently, but I've updated it to do the temporary coercion for both.

I think there's no performance impact since the temporary should get merged during codegen.