added Symreltab (binary relations of symbols) instance of TableFun

--- a/src/Pure/General/table.ML	Sat Mar 21 20:39:38 2009 +0100
+++ b/src/Pure/General/table.ML	Sun Mar 22 11:56:22 2009 +0100
@@ -383,3 +383,6 @@
 
 structure Inttab = TableFun(type key = int val ord = int_ord);
 structure Symtab = TableFun(type key = string val ord = fast_string_ord);
+structure Symreltab = TableFun(type key = string * string
+  val ord = prod_ord fast_string_ord fast_string_ord);
+