* Fixes the totally broken UserInfo resolution in guilds.
* Adds support for USER_UPDATE from the gateway.
Design considerations behind the user cache:
* Explicitly handle deleted user IDs by short circuiting the lookup
logic and returning a singleton.
* The cache map is protected during HTTP requests to the Discord API.
* The nonexistence of a user is cached. This is to prevent excessive
requests (a user can't suddenly begin existing at a given ID).
The user cache is upserted on READY, incoming messages, backfill, etc.
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