Some Changes for MergeSort

[Ccheers]

#18
It Make MergeSort faster than before and I Commit a pull request.
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Benchmark_MergeSort_new
Benchmark_MergeSort_new-12 10 101394033 ns/op 209956300 B/op 1000041 allocs/op
Benchmark_MergeSort_new-12 10 103156080 ns/op 209956320 B/op 1000041 allocs/op
Benchmark_MergeSort_new-12 10 105631558 ns/op 209956272 B/op 1000041 allocs/op
Benchmark_MergeSort_old
Benchmark_MergeSort_old-12 4 322263281 ns/op 582174710 B/op 4052123 allocs/op
Benchmark_MergeSort_old-12 3 333629871 ns/op 582175192 B/op 4052124 allocs/op
Benchmark_MergeSort_old-12 3 350259903 ns/op 582174706 B/op 4052122 allocs/op