Add async ready support

[kennykerr]

Building on #3213, this update adds support for creating async implementations that are immediately ready with a value. This is analogous to https://doc.rust-lang.org/stable/std/future/fn.ready.html and an acknowledgement that there's no need to spin up a thread or switch contexts if the results are already available - its way more efficient to just return them directly.

For example, let's say you need an IAsyncOperation<i32> that results in a value of 123. You can create one as follows:

use windows::{core::*, Foundation::*};

fn main() -> Result<()> {
    let ready = IAsyncOperation::ready(Ok(123));

    assert_eq!(ready.get()?, 123);

    Ok(())
}

The get call, which blocks the calling thread until the results are available, will return immediately with the expected value.