Support for mutations

[lhstrh]

• Refactor graph package to use "happens before" rather than "depends on" relationship (i.e., invert the arrows) (also see Graph Issues #166)
• Create online graph vizualizer (also see Graphical debugging of mutations #77)
• Implement addChild or addDescendent (Add relatives functionality for mutations, and fix Sieve #193)
• Implement addSibling or addNeighbor (ibidem)
• Fix Sieve (ibidem)
• Implement other benchmarks (Implement Savina benchmarks that require mutations #76)
• Figure out how to make the checks scale