aio: multishot operations

[Cloudef]

It's possible to have multishot operations that take a callback parameter. These are important for repeating timers and socket accept loops. Should not need any special interface, but there might be some lifetime shenanigans that has to be taken in account.

[Cloudef]

How it could work:

fn acceptCallback(context: *anyopaque, socket: std.posix.socket_t, addr: std.posix.addrinfo) !void {
    // cancel from inside a callback
    return error.Canceled;
}

var id: aio.Id = undefined;
try io.single(aio.AcceptMultishot{ .callback = acceptCallback, .context = arg, .out_id = &id });
// cancel from outside the callback
io.single(aio.Cancel{ .id = id });