Add push_multiple

[Kestrer]

Currently concurrent io_uring instances can't really be used to send linked SQEs because another thread could call push in between one thread calling push twice. This PR adds a new function, push_multiple, to allow pushing multiple SQEs atomically.

Since it's not easy to pass in an arbitrary number of owned values to a function in Rust yet, I decided that it would be better to make squeue::Entry Copy so that it wouldn't be inconsistent to take both Entry and &[Entry]. And since we no longer need to return back ownership of the Entry when the queue is full, the push functions return bools instead of Result<(), Entry> - although they could also return Result<(), ()>. Actually, after further thought I have decided to use Result<(), ()> because it has more obvious semantics and it is #[must_use].

[quininer]

I want to put it aside for now, until we decide on the future of concurrent module.

[quininer]

Unless the kernel supports it, it is difficult for us to provide meaningful concurrent mod.

This PR will allow developers to lock sq by themselves. maybe one day kernel will support concurrent queues, and then we can bring back concurrent mod. .

[quininer]

I'm considering adding this to normal sq and cq, it will have some performance advantages (batch submit see #65), and I find it useful when writing links like write + flush.