[WIP] Prototype for firm graph creation.

That's really just a rough first sketch, please let me know if something seems to be entirely wrong.
I checked the code with `Dump.dumpGraph` , the results seemed to be right. Works just for graphs with ints, +, *, and local variables of type int. See #127 and #124 
Don't forget to add `Firm.init()` and `Firm.finish()` somewhere...