Add the tiger source code bundle from the book site

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

tiger/chap9/graph.sig

@@ -0,0 +1,23 @@
+signature GRAPH =
+sig
+    type graph
+    type node
+    
+    val nodes: graph -> node list
+    val succ: node -> node list
+    val pred: node -> node list
+    val adj: node -> node list   (* succ+pred *)
+    val eq: node*node -> bool
+
+    val newGraph: unit -> graph
+    val newNode : graph -> node
+    exception GraphEdge
+    val mk_edge: {from: node, to: node} -> unit
+    val rm_edge: {from: node, to: node} -> unit
+
+    structure Table : TABLE 
+    sharing type Table.key = node
+
+    val nodename: node->string  (* for debugging only *)
+
+end