Model module cleanup and standardization

[vilgotf]

twilight-model has the following top level modules (with the modeled Discord doc categories):

• application - what Discord label as Interactions
• channel - what Discord label as Stage instance, Sticker and Webhook
• gateway - what Discord label as Gateway + our own custom gateway events
• guild - what Discord label as Audit Log, Emoji and Guild
• http - various HTTP models for twilight_http requests
• id - our ID optimizations and marker types
• invite - what Discord label as Invite and Guild (invite::WelcomeScreen)
• oauth - what Discord label as Application and Team
• scheduled_event - what Discord label as Guild Scheduled Event
• template - what Discord label as Guild Template
• user - what Discord label as User
• util - like id this contains other model optimizations

There's clearly a lot of inconsistency here, we're not following Discord's categorization and we're not nesting relevant things like template and scheduled_event under guild.
There's even collision with our application being totally different from Discord's Application category.

Since Discord has 15+ categories (depends on how you count them) I think it's unfeasible to model categories like Discord does. We should instead nest relevant modules as follows:

invite -> guild::invite
scheduled_event -> guild::scheduled_event
template -> guild::template

I also think application should be renamed to avoid confusion with oauth::Application and what Discord categorizes as Application, http to be expanded, channel::message::allowed_mentions::AllowedMentionsBuilder to be moved to util, id::marker to be flattened and id::Id to be exported, and guild::member to be hidden as the types there are internal.

To summarize: the model is large and scary for new users to the Discord API and to Twilight. By exposing only a few top level modules users are not immediately exposed to too much information, and information is easily searched for by logically exploring submodules' items (like messages belonging to channels and invites belonging to guilds). Long term we also need to document items; linking to Discord's docs and improving on them where necessary, including overarching guides for how Twilight fits together.