Add the tiger source code bundle from the book site

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

tiger/testcases/test6.tig

@@ -0,0 +1,13 @@
+/* define valid mutually recursive procedures */
+let
+
+function do_nothing1(a: int, b: string)=
+		do_nothing2(a+1)
+
+function do_nothing2(d: int) =
+		do_nothing1(d, "str")
+
+in
+	do_nothing1(0, "str2")
+end
+