UserspaceEmulator: Implement very basic leak checking :^)

Upon exit, the emulator will now print a leak report of any malloc
allocations that are still live and don't have pointers to their base
address anywhere in either another live mallocation, or in one of the
non-malloc-block memory regions.

Note that the malloc-block memory region check is not fully functional
and this will work even better once we get that fixed.

This is pretty cool. :^)

DevTools/UserspaceEmulator/Emulator.cpp

@@ -174,6 +174,10 @@ int Emulator::exec()
         if (trace)
             m_cpu.dump();
     }
+
+    if (auto* tracer = malloc_tracer())
+        tracer->dump_leak_report();
+
     return m_exit_status;
 }