Add HIPPO module and starting tracking free physical pages

src/hulk/hulk.d

@@ -11,6 +11,7 @@ import ldc.attributes;
 import hulk.kfont;
 import hulk.klog;
 import hulk.hurl;
+import hulk.hippo;
 
 extern extern(C) __gshared ubyte _hulk_total_size;
 
@@ -36,6 +37,8 @@ void hulk_start()
 
     klog.writefln("Welcome to HULK, the HOS UltraLight Kernel!");
 
+    hippo.initialize(&hulk_header);
+
     for (;;)
     {
     }