change postingsList type

[Shinichiro Abe]

before: List<int> -> after: Set<int>

indexing speed showed 70x faster from benchmark:

* before
1st
IndexSearcher(RunTime): 173250.0 us.
IndexWriter(RunTime): 504745000.0 us.
2nd
IndexSearcher(RunTime): 159923.07692307694 us.
IndexWriter(RunTime): 507978000.0 us.

* after
1st
IndexSearcher(RunTime): 160461.53846153847 us.
IndexWriter(RunTime):  7198000.0 us.
2nd
IndexSearcher(RunTime): 171416.66666666666 us.
IndexWriter(RunTime): 7181000.0 us.

[Shinichiro Abe]

fix #44

→ <<cset 32f581faa549>>

[Shinichiro Abe]

my powerful machine:

the indexing speed became 60x faster.

* before
1st
IndexSearcher(RunTime): 28166.666666666668 us.
IndexWriter(RunTime): 99754000.0 us.

2nd
IndexSearcher(RunTime): 27739.72602739726 us.
IndexWriter(RunTime): 100563000.0 us.

* after
1st
IndexSearcher(RunTime): 28800.0 us.
IndexWriter(RunTime): 1668500.0 us.
2nd
IndexSearcher(RunTime): 28871.428571428572 us.
IndexWriter(RunTime): 1664500.0 us.

[Shinichiro Abe]

fix #44. fix type at index searcher

→ <<cset 6c3b3ac50c54>>