Some helpful functions for `Media::Photo`, `Media::Document` and `PhotoSize`.

[hlf20010508]

• Support size() for Media::Photo to get the real size of the photo.
• Support thumbs() for Media::Document.
• Support download to memory for thumb.
• Support get specific type of photo thumb in VecExt.

[hlf20010508]

Ok, I've completely migrated download methods in PhotoSize to download_media and iter_download.

The usage is the same as other media

let downloadable = Downloadable::PhotoSize(photo_size);
let mut download = client.iter_download(&downloadable);

I also implemented download for PhotoSize::Progressive. It's actually the same as PhotoSize::Size. Now users can use largest() to get the largest thumb(for photos it's usually progressive) and download it.

And there was a bug in the download() of PathSize, that the param path was shadowed by the variable mut path. But it's not needed anymore.

[hlf20010508]

The new function DownloadIter::new_from_photo_size is ugly, since request is not needed, I have to fake a useless one. If you think it's ok to make request to Option<tl::functions::upload::GetFile>, I'll do that.

[Lonami]

From a quick look, as long as it gets the job done, then it's fine.