Introduce DependencyGraph, computing dependencies of elements.

The dependencies are computed through a list of blocs (IRA).
APIs .get* return an iterator on DependencyResult. Each one contains only relevant DependencyNode, which stand for an element at a given line in a given basic block. That way, outputs contain each elements involved in the targets' value computation.

Different outputs stand for different path through blocks (loop, if-then-else statements, ...).

In addition, the DependencyResult class offers some API, such as:

• a DiGraph output
• the symbolic execution of intermediary elements in order to get their expected value

By defining explicit dependencies as dependencies only involved by instruction semantics (unlike those involved by branching), the sub-algorithm NoMemory seems sound and complete.
The standard one (best effort) suffers from memory aliases.

This algorithm has been co-developped with @serpilliere .

An example of use is joined to this PR, through an IDA plug-in.

For instance, the source code of example/samples/simple_test.bin is (example/samples/simple_test.c):

int test(unsigned int argc, char** argv)
{
        unsigned int ret;
        if (argc == 0)
                ret = 0x1001;
        else if (argc < 2)
                ret = 0x1002;
        else if (argc <= 5)
                ret = 0x1003;
        else if (argc != 7 && argc*2 == 14)
                ret = 0x1004;
        else if (argc*2 == 14)
                ret = 0x1005;
        else if (argc & 0x30)
                ret = 0x1006;
        else if (argc + 3 == 0x45)
                ret = 0x1007;
        else
                ret = 0x1008;
        return ret;
}

Let's invoke the script on the return ret; statement (offset 0x88).

Once launch is pressed, a first path is highlight.

Line dependencies are added as comments (here the stack, and the constant 0x1003).

In addition, on the console (we track EAX):

Get graph number 01
Dump the graph to /tmp/solution_0x00000088_01.dot
Possible value: 0x1003

By pressing Shift+N, one can get every solutions one after one:

Possible value: 0x1002
Possible value: 0x1008
...
Possible value: 0x1001
Done: 8 solutions

Regression tests are based on 10 graphs with different layout.