datalog: update examples

[dhalperi]

A few suggestions:

1. All example queries should run
2. We should have a few useful ones front-loaded. Right now we focus on simple language use cases, but we should add examples that we know highlight profiling skew.

@stechu you've been the one most actively using Datalog. (I don't use it, at all.) Can you please take a pass on the examples?

[dhalperi]

@stechu can you take a stab at this? A good set of examples will:

• all be parseable
• all be runnable
• all be quick
• flesh out the space nicely.