Issue 49

[alldroll]

Our storage format for NGramVectors (see below) forces us to create an individual roaring.Bitmap object for each parent. It means, if we have 1kk 1-grams, for 2-grams we will have 1kk Bitmap instances, which obviously is not performant as we use the only feature Rank(x uint64).

[]uint64{
    ...
    parentID << 32 | wordID      // New York (Parent | Word)
    ...
}

So, this PR closes #49