mostly for cleaner code and better documentation

Adds CallSource nodes to the control flow graph, which have the same role for backwards analysis as the CallReturn nodes have for forward analysis.

which also enables Ghidra 9.2 support.

Now subregisters are accessed using SUBPIECE instructions. This should fix the issue that we could not assign subregister accesses to the corresponding base register in some cases.

adds nice explanatory images to the documentation of `graph.rs`.