Address arithmetic

Added example for address arithmetic demo using alloc

ch_05/alloc_demo/alloc.h

@@ -0,0 +1,10 @@
+
+#define ALLOCSIZE 9500
+
+static char allocbuf[ALLOCSIZE];
+static char *allocp = allocbuf;
+
+
+char *alloc(int n);
+
+void afree(char *p);