Eris -> Oceanic

[eritbh]

Base this lib on Oceanic rather than Eris. Their APIs are very similar, and Oceanic is written in TS which means we'll have reasonable types much more quickly than before, which should simplify a lot of development that was blocked under Eris.

If this works out cleanly and is a relatively simple migration for most people, this probably means we can un-maintenancemode Yuuko and start working on stuff like slash command support again.