rank moves without sorting the entire list

[niklasf]

Before:

[info] Benchmark               Mode  Cnt     Score    Error  Units
[info] HuffmanPgnBench.encode  avgt  120  7181.847 ± 48.616  us/op

After:

[info] HuffmanPgnBench.encode  avgt  120  3126.608 ± 19.164  us/op

[niklasf]

The full sort in encoding is even more useless ... intending to land this before #25, where there's more things to try, and to rename selectNth(n: Int) to something like selectRank(rank: Int).