Refactoring and overhaul of IACFG-generating code.

• joinCFGs: overhauled to simplify using the CallGraph.
• extractCalls: split into enter/exit and calls, using a recursive algorithm and linked lists of nodes.
• Increased project JDK level to 21 (latest LTS).
• Testing: moved main to src/test/java and Test.java to src/test/resources.
• Renamed IACFG to ICFG for simplicity.