added table functor instance for pairs of strings

--- a/src/Pure/General/table.ML	Wed Oct 19 17:19:52 2005 +0200
+++ b/src/Pure/General/table.ML	Wed Oct 19 17:21:53 2005 +0200
@@ -389,3 +389,7 @@
 
 structure Inttab = TableFun(type key = int val ord = int_ord);
 structure Symtab = TableFun(type key = string val ord = fast_string_ord);
+structure PStrStrTab = TableFun(
+  type key = string * string
+  val ord = prod_ord fast_string_ord fast_string_ord
+);