IntSeries / IntMutableList for joins and filters

[andrus]

Let's create IntMutableList (an appendable collection of primitive "int" values) that can be converted to IntSeries, which is immutable.

While working with collections of primitives in Java is painful, there can be real performance gains. My prototype of the data structures above speeds up joins by ~ 25-30% when used for indexing joined DataFrames.

This task will switch joins and filters to int-based implementation. Sorters and groupers will be switched separately, as this requires our own custom sorter.

[andrus]

Note that an implementation for joins is fairly straightforward. However an implementation for "sort" operation is more quirky, as JDK libs do not support sorting of int[] with a custom Comparator. Will need to write our own sort algorithm.

[andrus]

Latest performance measurements:

• Hash joins: 21-23% faster
• Nested loop joins: 2-14% slower (why ?!!)
• Filter: 37% faster

[andrus]

After related #27 implementation, the numbers are improved:

Latest performance measurements:

Hash joins: 33-35% faster
Nested loop joins: 3-13% slower
Filter: 37% faster