Improve memory usage for anonymous variables

[stariy95]

While #4 fixes leaks linked to a named variable redefinition, there's still more options for improvement.
The first candidate is anonymous variables:

DataFrame
        .foldByColumn("1", "2")
        .ofStream(IntStream.range(0, Integer.MAX_VALUE / 8));

Expression above creates different snippets each time and they are not linked, like in case of a named variables, so no class unloading can happen.