First Cut at GroupVarInt with FixedIntBlockIndexInput / Output [LUCENE-2735]

I have hacked together a FixedIntBlockIndex impl with Group VarInt encoding - this does way worse than standard codec in benchmarks but I guess that is mainly due to the FixedIntBlockIndex limitations. Once #3797 is in / or builds with trunk again I will update and run some tests. The isolated microbenchmark shows that there could be improvements over vint even in java though and I am sure we can make it faster impl. wise.

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Migrated from [LUCENE-2735](https://issues.apache.org/jira/browse/LUCENE-2735) by Simon Willnauer (@s1monw), updated May 09 2016
Attachments: [LUCENE-2735_alt.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2735/LUCENE-2735_alt.patch), [LUCENE-2735.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-2735/LUCENE-2735.patch) (versions: 3)