caching private channels

[laralove143]

Guild channels are cached on ChannelUpdate events but creating a private channel doesn't fire any events so they're not cached, this means we would either have to cache it manually or make an avoidable API request every time.

If this is approved I can try to PR it, given the method of caching.

Since there won't be any events, it'd have to be made like

let channel = http.create_private_channel(user_id).exec().await?.model().await?;
http.create_message(channel.id).content("hello")?.exec().await?;
cache.update(channel);

Another consideration is invalidation, if the user closes the DM channel, will it be an API error to try to use it? Does Discord send any events for closed channels? If not, how can we handle invalid channels?