reduce cascading Tuple & TupleEntry copies

[johnynek]

According to @ianoc 's benchmarks it can be 5-8%.

See:

https://gitter.im/twitter/scalding?at=56b4eda42079fb5f156f1697

there are APIs that should allow the code in Operations to keep one private instance and reuse. In the case of hash keys, they are almost certainly single items. In that case, just pull them out.