[WIP] init commit isomorphism invariant hash codes

[bbrehm]

This brings in an isormorphism invariant hashcode for cfgNodes, especially METHOD.

quote docstring:

"An isomorphism-invariant hash-code that describes a CfgNode and its position inside the method body. The ISOHASH of METHOD nodes can be used to quickly heuristically check whether a method has changed between two CPGs emitted by the same frontend. Thw ISOHASH of cfgnodes can be used as a stable (albeit collision-prone) way to identify a specific node within a method. Line-numbers, filenames, etc are intentionally excluded from the hash computation. If a method body contains automorphisms, then one will get stable collisions: In e.g. if(condition) foo(); else foo();, the two foo() calls are indistinguishable. The ISOHASH is not intended to be stable under varying frontend versions; it e.g incorporates names of local variables (which can be a frontend lowering decision, when e.g. translating a DUP instruction from a stack machine). The ISOHASH is not designed to be used to detect differences from unreasonable frontend changes (i.e. bugfixes or bug introductions). The 8 most significant bits mark the version of the hash computation algorithm, and the lower 56 bits should look like pseudo-random."