minor additions to the kernel heap and adding acpi

kernel/heap.h

@@ -3,6 +3,9 @@
 
 #include <stddef.h>
 
+void heap_init(void *heap_start, void *heap_end);
+
 void *heap_alloc(size_t size);
+void heap_free(void *ptr);
 
 #endif // HEAP_H