Fixed bugs about kheap, paging and ordered list

Furthermore, added Heap kernel driver code
for both allocation and deallocation
free() does not works yet:
`ASSERTION-FAILED(head->magic == HEAP_MAGIC) at kheap.c:261`

kernel/drivers/kheap.c

@@ -251,7 +251,7 @@ void *alloc(uint32_t size, uint8_t page_align, heap_t *heap) {
 
 void free(void *p, heap_t *heap) {
     // Exit for null pointer
-    if(p == NULL)
+    if(p == 0)
         return;
     
     // Retrieve the header and the footer for this pointer