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/MallocTracer.h

@@ -44,6 +44,8 @@ public:
     void audit_read(FlatPtr address, size_t);
     void audit_write(FlatPtr address, size_t);
 
+    void dump_leak_report();
+
 private:
     struct Mallocation {
         bool contains(FlatPtr a) const
@@ -57,8 +59,11 @@ private:
     };
 
     Mallocation* find_mallocation(FlatPtr);
+    bool is_reachable(const Mallocation&) const;
 
     Vector<Mallocation> m_mallocations;
+
+    bool m_auditing_enabled { true };
 };
 
 }