Add generic dataflow framework + example + stack value analysis

[montyly]

• Add AbstractDataflow, which is a generic dataflow analysis, following an abstract interpretation approach.
• Add CollectInstruction, which is just an example. The analysis collects all the instructions executed
• Add StackValue, which track the values push/pop on the stack, with a focus on int/global field/transaction field

Build on top of 116

This is a draft. Todo list:

• Implement missing opcode handlers for important operations
• Add more tests (ex: to stress the merging operations)

Once this is done, the next step will be to increase the abstract value to also track light equation (ex: Eq(1, GroupSize)), and propagating them while collecting logical operations (or, and)

[S3v3ru5]

is this supposed to be

while values and values[0] == StackValue(Top()):
    values.pop(0)

Or

while values and values[-1] == StackValue(Top()):
    values.pop()

I'm guessing it's the first one (?)

[S3v3ru5]

Shouldn't this line be poped_values = self.values[-number:] (?)

[S3v3ru5]

Shouldn't this lines be

v1 = v1[-min_length:]
v2 = v2[-min_length:]

[S3v3ru5]

Stack union depends on the length of inputs.

    def union(self, stack: "Stack") -> "Stack":

        v1 = self.values
        v2 = stack.values

        min_length = min(len(v1), len(v2))
        v1 = v1[:-min_length]
        v2 = v2[:-min_length]

        v3 = []
        for i in range(min_length):
            v3.append(v1[i].union(v2[i]))

        return Stack(v3)

Because initial s = Stack([]) has length 0, output of union operation will be empty stack. _merge_predecessor will always result in empty stack.
Correct approach would be:

if not bb.prev: # zero predecessors
    return Stack([])

s = self.bb_out[bb.prev[0]]
for bb_prev in bb.prev[1:]:
    s = s.union(self.bb_out[bb_prev])

return s

[S3v3ru5]

^^ Won't this approach result in errors when transaction fields from gtxn instruction are tracked alongside txn instruction?

1. txn RekeyTo
2. gtxn 3 RekeyTo

RekeyTo from 1 and 2 would have same str(self) and type. They will be considered equal during stack value analysis. txn RekeyTo and gtxn 3 RekeyTo does not necessarily be equal and most of the times will not be equal.

I think we need a better way to compare and hash TransactionField objects.