Added memset utility in string.h

src/string.c

+#include "string.h"
+
+void *memset(void *s, int c, size_t n){
+  unsigned char* p=s;
+  while(n--)
+    *p++=(unsigned char) c;
+  return s;
+}

src/string.h

+#ifndef _STRING_INC
+#define _STRING_ING
+#include<stddef.h>
+
+void* memset(void *s, int c, size_t n);
+#endif