Make sieve be local, not global.

bench-sieve.txt

@@ -9,7 +9,7 @@ evals = evaluations();
 
 for (n = 0;  n < repeats;  n = n + 1) {
     print("\r", n, "/", repeats);
-    sieve = [];
+    let sieve = [];
     count = 0;
     for (i in numbers) sieve.push(nil);
     for (i = 2; i < numbers;  i = i + 1) {