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.

Code and documentation improvements for all abstract domains

Insert missing return statements into the control flow graph.

Added brand new (and still experimental) checks for CWEs 415 and 416 together with a new interprocedural data-flow analysis engine written in Rust. Add `-partial=Memory` as command line flag to try out the new checks.