Def-use analysis for parsers and controls

[mihaibudiu]

Detection of uninitialized variables and fields built on top of def-use analysis.
testdata should not be in .gitignore

[ChrisDodd]

When I look at this code, it seems like it is duplicating most of the functionality of ControlFlowVisitor and P4WriteContext, so it seems like it could be simplified by using those Visitor mixins.

[mihaibudiu]

There are some similarities, but I have a hard time wrapping my head around the way these mixins split the work among themselves. There is only one example in the front-end, the DoLocalCopyPropagation, which does not handle many statements, and seems to be broken currently (it does nothing, in part because it is run at the wrong point in the pass sequence).

Also, the def-use analysis handles loops in parsers and does inter-procedural analysis for controls (invoking tables and actions). It's conceivable that the visiting part could be refactored into a subclass of ControlFlowVisitor, and perhaps we should consider doing this at some point.