add the async Iterator::filter method

[yoshuawuyts]

This adds an async closure-based filter method. It compiles on the latest nightly, but R-A doesn't like it one bit yet so marking this as a draft PR. This does show that the we can correctly implement async closure-based APIs using an AFIT-based async Iterator trait.

[yoshuawuyts]

🎉

[compiler-errors]

It would be useful if there were a test committed that actually shows how this is more powerful than StreamExt::filter. Specifically, maybe something like:

fn test_filter_fn(x: impl crate::Iterator<Item = String>) -> impl crate::Iterator<Item = String> {
    async fn do_a_query(_: &str) -> bool { true }
    x.filter(async |url: &String| do_a_query(url).await)
}

Note that async closures don't infer their signatures yet, so you need to write |url: &String| in the signature, or else you'll get inference errors.

[compiler-errors]

but R-A doesn't like it one bit yet so marking this as a draft PR.

rust-lang/rust-analyzer#16588