A10 on stable Rust

[Thomasdezeeuw]

Below is a list of nightly features used in order of (roughly) to-be-solved-first to to-be-solved-last/easiest-to-solve-now, in other words the ones at the top are harder to solve so start with those (no point in losing functionality if we're still on nightly for another feature).

• slice_ptr_get: used in various places.
• ptr_sub_ptr: only used in ReadBuf::release.
• maybe_uninit_array_assume_init: used in the BufMutSlice implementation for arrays and tuples.
• maybe_uninit_uninit_array: used in the BufMutSlice implementation for arrays and tuples.
• io_error_more: used in the From<QueueFull> impl for QueueFull, could use io::ErrorKind::Other for the time being.
• async_iterator: used in multishot futures, could be replaced with a separate poll_next method.
• new_uninit: used in a couple of Futures, could use Box::new(MaybeUninit::new()), although I think that's less efficient.
• const_cmp: only used in Config::with_idle_timeout, could be made non-const.
• const_mut_refs: used in Submission::set_user_data and Submission::set_buffer_select, could be made non-const.
• mutex_unlock: used in SubmissionQueue::add_no_result and ReadBuf::release, can easily be replaced with a call to drop (and maybe a comment).