Fix bug with dataset iterator processing

[khatchad]

• Fix bug with iterating over datasets whose elements may be tuples.
  • This happens when a tensor dataset generator outputs tensors in a specific format.
  • Previously, Ariadne was not inferring tensors that were being picked out of the tuple during dataset iteration.
  • Added corresponding tests.
• Don't consider exceptions as tensor dataflow sources.
  • This was happening in the old code. I've added it to the new code as well.
• Since invocation instructions are processed in multiple places, I've extracted a common method.
  • The two different call sites differ on the src points-to variable to be added.
  • This guarantees that the order in which the points-to variables are processed doesn't matter.