An implementation of the Discord API

Goals:

• Remove as much partiality as possible. ExceptT instead of IO exceptions, no error, etc etc
• Type-safe API similar to Servant (likely: use Servant for backend)
• Caching support (with proper isolation of servers)
• Eventually: pluggable caches (in-memory, Redis, ...)
• Ideally pluggable events --- user should be able to add a missing endpoint or whatever. Probably not reasonably possible.
• Proper separation of servers (guilds) --- user data should not leak from one to the other
• Websocket stuff, eventually voice
• Storage backends are probably out of scope for this?
• Honor rate limits
• Properly handle all discord errors
• Properly handle authentication. Eventually: Full OAuth2 flow support
• Be compatible with an eventually consistent system

Never be sent to a client
Be sent exactly one time to the client
Be sent up to N times per client
Clients should operate on events and results from the API in as much of a idempotent behavior as possible.

• Properly handle user-agent