Support Compilation on Stable Rust

[kaimast]

This crate uses a bunch of unstable features right now that requires it to be built on nightly. We either need to wait for them to stabilize or rewrite the code in this crate to stop using them.

• Tracking issue for map_first_last: first/last methods on BTreeSet and BTreeMap rust-lang/rust#62924
• Tracking issue for trait aliases rust-lang/rust#41517
• Tracking Issue for #![feature(async_iterator)] rust-lang/rust#79024
• Tracking issue for Write::write_all_vectored rust-lang/rust#70436
• Tracking Issue for array_methods rust-lang/rust#76118
• Tracking issue for {Rc, Arc}::get_mut_unchecked rust-lang/rust#63292
• Tracking issue for io_slice_advance rust-lang/rust#62726
• Tracking Issue for box_into_inner rust-lang/rust#80437