Add the tiger source code bundle from the book site

Signed-off-by: jmug <u.g.a.mariano@gmail.com>

tiger/testcases/test18.tig

@@ -0,0 +1,15 @@
+/* error : definition of recursive functions is interrupted */
+let
+
+function do_nothing1(a: int, b: string):int=
+		(do_nothing2(a+1);0)
+
+var d:=0
+
+function do_nothing2(d: int):string =
+		(do_nothing1(d, "str");" ")
+
+in
+	do_nothing1(0, "str2")
+end
+