Remove AbstractEvent and AbstractVenue

Been wanting to do this for while. The `SourceParser` subclasses were creating an `AbstractEvent` and an `AbstractVenue`, but then immediately invoking complex methods to convert them to a normal `Event` and `Venue`. The abstract versions were transient, and never used outside of this process, and thus unnecessary. This refactoring removes all that complexity, and now `SourceParser` subclasses create `Event`s and `Venue`s directly.

This exposes more low-hanging refactoring fruit among the `SourceParser` subclasses, but I'll save that for later.
Okay confirmed that changing `.collect` to `.lazy.collect` in your code snippet passes the tests on 2.0, 2.1, but not 1.9. Hmm. Did you mention that there are plans to drop 1.9 support?