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.

[jgardn3r]

I didn't respond to each review comment because many things have changed and it is practically a rewrite. I have tried to:

1. Improve documentation and explanatory comments
2. Use immutability
3. Create an API that isn't public

[jgardn3r]

There are a few aspects of the current shape that I am not completely happy with yet that I think would be worth discussing/brainstorming:

• The modelling of the successors
  • getSuccessors()
    • Returns an interface that allows you to check what kind of successors there are
    • Is this worth it?
    • This replaces the sonar-java approach of having a flag for a bunch of different things, e.g., isFinallyBlock, isCatchBlock
  • getSuccessorBlocks()
    • This is the successor-type agnostic way of getting all of the successor blocks
    • Generically useful for traversing blocks in rules, etc.
  • Object reference permanence - not necessarily a bad thing, just worth noting
    • When constructing the blocks the successors are treated as immutable
    • If there is a change to the successors a new successor object is used to replace
    • Blocks reference each other using address, rather than ID
  • An idea I had is only making the strongly coupled object as the end and using IDs internally

[fourls]

Also, I'm not sure if you've noticed @jgardn3r but there are still a few unresolved conversations from @Cirras 's first round of review.

[jgardn3r]

Went a little overzealous with the delete button and failed to compile again. All @Cirras comments should be addressed now.