hegg v0.2

What's Changed

• Expose runEqualitySaturation to run equality saturation on existing e-graphs
  whole instead of focusing on individual expressions
• (Very) significant performance improvements!
• Make CostFunction polymorphic over the Cost type, requiring that type
  to instance Ord
• Make e-graph abstract. The internal structure can still be modified through
  the available lenses in Data.Equality.Graph.Lens
• Fix a bug related to NodeMap's size.

Full Changelog: v0.1.0.0...v0.2.0.0