Move memory_manager and assert into kutil.

src/kernel/kalloc.h

@@ -0,0 +1,17 @@
+#pragma once
+/// \file kalloc.h
+/// Definitions of kalloc() and kfree()
+
+#include <stddef.h>
+
+
+/// Allocate kernel space memory.
+/// \arg length  The amount of memory to allocate, in bytes
+/// \returns     A pointer to the allocated memory, or nullptr if
+///              allocation failed.
+inline void * kalloc(size_t length);
+
+/// Free kernel space memory.
+/// \arg p   The pointer to free
+inline void kfree(void *p);
+