Implement topk() and bottomk() functions.

To achieve O(log n * k) runtime, this uses a heap to track the current
bottom-k or top-k elements while iterating over the full set of
available elements.

It would be possible to reuse more code between topk and bottomk, but I
decided for some more duplication for the sake of clarity.

This fixes https://github.com/prometheus/prometheus/issues/399

Change-Id: I7487ddaadbe7acb22ca2cf2283ba6e7915f2b336