Add support for for await loops

[eholk]

This adds support for for await loops. This includes parsing, desugaring in AST->HIR lowering, and adding some support functions to the library.

Given a loop like:

for await i in iter {
    ...
}

this is desugared to something like:

let mut iter = iter.into_async_iter();
while let Some(i) = loop {
    match core::pin::Pin::new(&mut iter).poll_next(cx) {
        Poll::Ready(i) => break i,
        Poll::Pending => yield,
    }
} {
    ...
}

This PR also adds a basic IntoAsyncIterator trait. This is partly for symmetry with the way Iterator and IntoIterator work. The other reason is that for async iterators it's helpful to have a place apart from the data structure being iterated over to store state. IntoAsyncIterator gives us a good place to do this.

I've gated this feature behind async_for_loop and opened #118898 as the feature tracking issue.

r? @compiler-errors