Add control flow graph implementation

[jgardn3r]

This PR adds the notion of a control flow graph. Control flow graphs are a stepping stone in data flow analysis and symbolic execution.

With just the control flow graph alone, two new rules could be implemented:

• RedundantJump
• LoopExecutingAtMostOnce

In addition to that, some more API traversal methods were added:

• RepeatStatementNode::getGuardExpression
• RepeatStatementNode::getStatements
• CaseStatementNode::getSelectorExpression
• CaseItemStatementNode::getStatement

The thinking is that the control flow graph will be internal for a while before being exposed in the stable API. In light of this, I have made a ControlFlowGraph interface in the API, but there are no public ways to create/interact with it.