Method to upgrade a REST object to a Gateway object

[A5rocks]

Summary

A method that upgrades from a REST object to a Gateway object.

Problem

Some of the Gateway objects are better (see for example is_large on GatewayGuild, etc.) However, to convert a REST object to a Gateway one, you need to use cache methods. That's annoying.

Ideal implementation

RESTXXX.upgrade(self) -> Optional[GatewayXXX]

Just calls the respective cache methods.

Checklist

• I have searched the issue tracker and have made sure it's not a duplicate.
  If it is a follow up of another issue, I have specified it.

[A5rocks]

Now that I check it out, only RESTGuild and GatewayGuild have this distinction, this feature request might not be worth it.

However, there's some non-RESTXXX things that are not on REST -- take for example, https://hikari-py.github.io/hikari/hikari/messages.html#hikari.messages.Message.guild_id. Maybe a Message.upgrade(self) -> Optional[Message] would be nice?

[davfsa]

After seeing issues like this one and #582 or #587 I am starting to think our models are a bit of a mess and should be restructured as well as clearly documented the steps of action for new models

cc @FasterSpeeding

[FasterSpeeding]

This'd likely be called get_self not upgrade to fit in with our the format for fetch vs get methods and to match the current fetch_self method naming standard present in the lib. Also would probably be a method on partial guild with rest guild inheriting it

[FasterSpeeding]

After seeing issues like this one and #582 or #587 I am starting to think our models are a bit of a mess and should be restructured as well as clearly documented the steps of action for new models

Most of the models seem fine to me, #582 mostly just changes events with the only models within its scope being the guild and message models which funnily enough are also the only models relevant to this and #587. Message and guild do unique stuff which isn't really representative of how models are handled in their current state.