Added count sketch

[c-dickens]

Added a python implementation of count sketch for frequency estimation and heavy hitter detection.

[jmalkin]

There's a valid philosophical debate over whether adding and then subtracting an item means a sketch is truly empty. It might seem trivial with a single item, but imagine merging 2 sketches that were created with the same items but opposite weights -- if we treat it as empty, we lose the information that it has ingested data.

Certainly open to the idea that we don't care about a distinction, but it's worth thinking about what we want to capture by "empty"

[jmalkin]

This is a reasonable start, but I think there's still a fair amount to do before it's production ready.

• there's no serialization, so it's not yet suitable for distributed use
• merge doesn't check for sketch compatibility, including matching table sizes. in order to be meaningful, I think we'd also want the hash functions to match?
• we really need to figure out a better hash function, as that seems important for ingesting arbitrary(-ish, in python) input types.

But I think this is sufficient to pull into a branch so we can start collaborative improvements