In this change, if the postings/postingsLocs slices need to be grown,
then copy over and reuse any of the preallocated roaring Bitmap's from
the old slice.
AnalysisResultsToSegmentBase() allows analysis results to be directly
converted into a zap-encoded SegmentBase, which can then be introduced
onto the root, avoiding the creation of mem.Segment data structures.
This leads to some reduction of garbage memory allocations.
The grouping and sorting and shaping of the postings list information
is taken from the mem.Segment codepaths.
The encoding of stored fields reuses functions from zap's merger,
which has the largest savings of garbage memory avoidance.
And, the encoding of tf/loc chunks, postings & dictionary information
also follows the approach used by zap's merger, which also has some
savings of garbage memory avoidance.
In future changes, the mem.Segment dependencies will be removed from
zap, which should result in a smaller codebase.
Steve Yen