Initial code dump
Import FastUtil
Add IQuantileEstimator directly, remove old imports
Implement our own QuickSelect algorithm
Add upstream headers, remove redundant copies of Guava classes
Fix build.
Register percentile streams.
Add the ability to set percentiles and the execution hint to PercentileBuilder.
cleanup of the percentiles aggregation
Fix infinite loop.
Add (failing) tests.
Fix NPE in Frugal.Merger.merge.
Remove the dependency on fastutil, use hppc instead.
Use thread-local randoms.
Improve Frugal merging.
Ensure Frugal estimates are within the interval of visited values.
Basic benchmark (accuracy & performance) for percentile aggregations.
Improvements to TDigest:

 - the histogram is now stored using parallel arrays instead of a tree of objects
 - use of a red-black tree instead of an AVL tree
 - various changes to make it faster (avoid computing things twice, update in place instead of remove-then-add)
Import accuracy tests from t-digest.
Remove object allocations from TDigestState.add.

This improves performance by ~10% according to the benchmark.