As planed for a long time, node properties are now accessed using constants rather than magic strings. The point of this change is to make visible which property keys currently exist, allowing people new to the code base to get a quick overview. It's also the first step in minimizing the amount of required keys.
Adaption of code to use of NodeKeys constants complete.
Since we now have gotten rid of basic blocks, we don't need USE/DEF links both from the AST and the CFG as they are the same merged structure.
Improved last commit: it's nice to have USE/DEF links from AST nodes. We just don't want duplicate links.