libcore: add a trivial uint hash function.

src/libcore/uint.rs

@@ -103,6 +103,13 @@ pure fn ge(x: uint, y: uint) -> bool { ret x >= y; }
 /* Predicate: gt */
 pure fn gt(x: uint, y: uint) -> bool { ret x > y; }
 
+/*
+Function: hash
+
+Produce a uint suitable for use in a hash table
+*/
+fn hash(x: uint) -> uint { ret x; }
+
 /*
 Function: range