Expose multiple HyperLogLog estimators

[fsaintjacques]

• original estimator with elementary alpha correction
• [google] proposed interpolation
• [lookup] table
• best fitting [polynomial]

[google] Stefan Heule, Marc Nunkesser, and Alexander Hall. Hyperloglog in practice: Algorithmic
engineering of a state of the art cardinality estimation algorithm. In
Proceedings of the 16th International Conference on Extending Database Technology,
pages 683–692, Genoa, Italy, March 2013.

[lookup] Lee Rhodes. System and method for enhanced accuracy cardinality estimation,
September 24 2015. US Patent 20,150,269,178.

[polynomial] Salvatore Sanfilippo. Redis new data structure: The HyperLogLog. http:
//antirez.com/news/75, 2014.