Kernel: Implement basic module unloading :^)

Kernel modules can now be unloaded via a syscall. They get a chance to
run some code of course. Before deallocating them, we call their
"module_fini" symbol.

Kernel/TestModule.cpp

@@ -1,7 +1,5 @@
 #include <Kernel/kstdio.h>
 
-extern "C" void outside_func();
-
 extern "C" void module_init()
 {
     kprintf("TestModule has booted!\n");
@@ -10,3 +8,8 @@ extern "C" void module_init()
         kprintf("i is now %d\n", i);
     }
 }
+
+extern "C" void module_fini()
+{
+    kprintf("TestModule is being removed!\n");
+}