aio: add aio.Poll

[Nyaa97]

Add of the poll op described at https://unixism.net/loti/ref-iouring/io_uring_enter.html

[Nyaa97]

Poll events should be bit map
But I don't see myself using a single Pool for several events

pub const Poll = struct {
    pub const Events = packed struct(u32) {
        in: bool = false,
        out: bool = false,
        pri: bool = false,
        _: u29 = 0,
    };

    pub const Error = std.posix.PollError || SharedError;
    fd: std.posix.fd_t,
    events: Events = .{ .in = true },
    out_id: ?*Id = null,
    out_error: ?*Error = null,
    link: Link = .unlinked,
    userdata: usize = 0,
};


aio.Single(aio.Poll{ .fd = fd, .events = .{ .out = true } });

In addition, I don't like the implementation of Readiness.mode which makes it difficult to simply check mode

pub const Readiness = struct {
    fd: std.posix.fd_t = invalid_fd,
    mode: packed struct(i16) {
        in: bool = false,
        out: bool = false,
        pri: bool = false,
        kludge: bool = false,
        _: u12 = 0,
    },
};

I made a draft
I am not sure how it should be 🤔

[Cloudef]

The readiness originally was not meant to implement poll. Your changes make sense. The kludge field probably can live outside of the packed struct as bool, as it is not a poll thing. Then I guess Events could be intCast'd to mode.

I'd also be grateful if you can add more tests.