Optimize Kuromoji inner loop - rewrite ConnectionCosts.get() method [LUCENE-3935]

I've been profiling Kuromoji, and not very surprisingly, method `ConnectionCosts.get(int forwardId, int backwardId)` that looks up costs in the Viterbi is called many many times and contributes to more processing time than I had expected.

This method is currently backed by a `short[][]`. This data stored here structure is a two dimensional array with both dimensions being fixed with 1316 elements in each dimension. (The data is `matrix.def` in MeCab-IPADIC.)

We can rewrite this to use a single one-dimensional array instead, and we will at least save one bounds check, a pointer reference, and we should also get much better cache utilization since this structure is likely to be in very local CPU cache.

I think this will be a nice optimization. Working on it...

---
Migrated from [LUCENE-3935](https://issues.apache.org/jira/browse/LUCENE-3935) by Christian Moen (@cmoen), updated Mar 30 2012
Attachments: [LUCENE-3935.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-3935/LUCENE-3935.patch)

Start at ConnectionCosts.get(int forwardId, int backwardId) and inspect its short[][] backing data alongside matrix.def. Compare the current representation with the proposed one-dimensional layout, then verify that Kuromoji’s Viterbi cost lookups still work and that the optimization improves the profiled hot path.

Written by the indexing model from the issue text.