Expose the guard API to allow users to enter tokio runtime

[ItsSunnyMonster]

I came across #21 which is similar to this issue but it seems like only the Drop part of that issue was implemented.

What I have is an enum which contains two variants: one contains a File from the async_fs crate and one contains a Compat<TempFile> from the async_tempfile crate (which uses tokio), and I have implementations of AsyncRead and AsyncSeek for this enum which just calls the poll_read function of the underlying types. I need to somehow enter the tokio runtime when the variant is the one containing the tempfile.

I could just document the fact that my type has to be used with compat, but this is not great for two reasons: the users would have to put compat everywhere they use my type, and also they would be unnecessarily entering a tokio runtime every single time the enum's variant is the one containing the async file.

Here's some code to better illustrate my point:

pub enum AssetFileReader<'r, R>
where
    R: AsyncRead + AsyncSeek + Unpin,
{
    // This "DirectFileReader" thing just holds a reference to an async File
    // It also forwards the async File AsyncRead and AsyncSeek implementations
    Normal(DirectFileReader<'r, R>),
    Decompressed(Compat<TempFile>),
}

impl<'r, R: AsyncRead + AsyncSeek + Unpin> AsyncRead for AssetFileReader<'r, R> {
    fn poll_read(
        self: Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
        buf: &mut [u8],
    ) -> Poll<futures_lite::io::Result<usize>> {
        let this = self.get_mut();

        match this {
            AssetFileReader::Normal(r) => Pin::new(r).poll_read(cx, buf),
            AssetFileReader::Decompressed(r) => {
                // This is perhaps what the API should look like?
                let _guard = enter_tokio_runtime();
                Pin::new(r).poll_read(cx, buf)
            },
        }
    }
}

// The AsyncSeek implementation is similar