feat(memory, string): add memory and string utilities

include/memory.h

@@ -0,0 +1,10 @@
+#ifndef RIVOS_MEMORY_H
+#define RIVOS_MEMORY_H
+
+#include "types.h"
+
+void *kmemset(void *buf, char c, size_t n);
+
+void *kmemcpy(void *dst, const void *src, size_t n);
+
+#endif