speed up isSet; more benchmarks

Running suite isSet [bench/isset.bench.js]...
>> isSet(unsortedBag) x 790,309 ops/sec ±0.75% (100 runs sampled)
>> isSet(unsortedSet) x 789,221 ops/sec ±0.70% (101 runs sampled)
>> isSet(sortedBag) x 786,332 ops/sec ±0.72% (98 runs sampled)
>> isSet(sortedSet) x 790,996 ops/sec ±0.26% (100 runs sampled)
Fastest tests are isSet(sortedSet),isSet(unsortedBag),isSet(unsortedSet) at 1.00x faster than isSet(unsortedBag)


Running suite isSet [bench/isset.bench.js]...
>> isSet(unsortedBag) x 3,572,194 ops/sec ±0.81% (101 runs sampled)
>> isSet(unsortedSet) x 3,606,429 ops/sec ±0.75% (99 runs sampled)
>> isSet(sortedBag) x 3,515,737 ops/sec ±1.46% (95 runs sampled)
>> isSet(sortedSet) x 3,678,495 ops/sec ±0.16% (103 runs sampled)
Fastest test is isSet(sortedSet) at 1.02x faster than isSet(unsortedSet)

Could make this much simpler, have a version local

```
isSet = function(collection) {
   for(var i = 0, length = collection.length; i < length; i++) {
      if (indexOf(collection, collection[i], i + 1) >= 0) return false;
   } 
   return true;
}
```