Refactor logic into separate classes

[schneems]

The frontier currently holds logic for "track lines we've not yet created blocks out of" as well as "evict blocks that are engulfed by larger blocks". These are two logically separate concerns that can be extracted into their own classes.

• Unvisited class, holds a hash of lines that have been visited and makes sure visited lines don't show up when pop is called
• Engulf class, a priority queue that also performs an engulf search and mark. When a block is marked as being deleted from the queue it will not show up in pop.

In addition there's a minor perf in the unvisited logic. We don't have to search and remove the element in the array, we can be lazy and remove it once it finds its way to the end of the queue since we're already tracking them in a hash. Also moved to make the hash compare_by_identity. Results are too close to post perf difference without doing some statistical analysis.