Tiny association and set refactors

As @jahfer and I were going through the association code, we cleaned up a few things in the areas we touched. This includes:

- whitespace
- `@` -> `this` where appropriate
- clearer naming

We also moved set event firing out of `SimpleSet` and into `Set` itself, which follows the pattern `Hash` uses.

@ibawt @BlakeMesdag for a quick review.