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.

[compiler-errors]

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

https://github.com/rust-lang/rust-analyzer/pull/16588