make ceylon.language::sum fast

src/ceylon/language/sum.ceylon

@@ -4,10 +4,33 @@ see (`function product`)
 shared Value sum<Value>({Value+} values) 
         given Value satisfies Summable<Value> {
     value it = values.iterator();
-    assert (!is Finished first = it.next());
-    variable value sum = first;
-    while (!is Finished val = it.next()) {
-        sum += val;
+    value first = it.next();
+    if (is Integer first) {
+        // unbox; don't infer type Value&Integer
+        variable Integer sum = first;
+        while (is Integer val = it.next()) {
+            Integer unboxed = val;
+            sum += unboxed;
+        }
+        assert (is Value result = sum);
+        return result;
+    }
+    else if (is Float first) {
+        // unbox; don't infer type Value&Float
+        variable Float sum = first;
+        while (is Float val = it.next()) {
+            Float unboxed = val;
+            sum += unboxed;
+        }
+        assert (is Value result = sum);
+        return result;
+    }
+    else {
+        assert (!is Finished first);
+        variable value sum = first;
+        while (!is Finished val = it.next()) {
+            sum += val;
+        }
+        return sum;
     }
-    return sum;
 }