0.8: completely overhaul event & content handling

- Big changes are afoot! This release decouples event metadata from event
  content, and decouples event content from the massive enum it once was.
- Also, we now actually check the event type instead of using untagged
  enums.
- Essentially, each possible matrix event now has its content struct
  somewhere in the types::content module.
- Documentation exists, too!