Move IntBlockPool's slice allocator and SliceReader/Writer out to MemoryIndex [LUCENE-10211]

[asfimport]

Spinoff from exciting issue #10712.

Lucene's ByteBlockPool and IntBlockPool are sort of like a little malloc implementation, embedding many logical growing int[] into a series of block buffers.  This is needed to compactly/efficiently store postings data, since there can be many unique terms, each with their own growing byte[] holding postings.

IntBlockPool also has these slices, however, nowhere in core do we use those.  Rather, the allocation needs for int[] is simpler: just allocated a fixed length 1, 2 or 3 int[] per unique term.  We can greatly simplify IntBlockPool.

However, MemoryIndex does use these slices from IntBlockPool.  I think we should move the complex slice logic in IntBlockPool out to MemoryIndex, simplifying core?

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Migrated from LUCENE-10211 by Michael McCandless (@mikemccand)