Use single nearest neighbor method instead of iterator

$ cargo benchcmp master single-nn --threshold 5
 name                         master ns/iter  single-nn ns/iter  diff ns/iter   diff %  speedup
 bench_all_to_all_parallel    19,344,342      15,956,802           -3,387,540  -17.51%   x 1.21
 bench_all_to_all_serial      165,671,156     137,606,946         -28,064,210  -16.94%   x 1.20
 bench_arena_query            601,230         433,101                -168,129  -27.96%   x 1.39
 bench_arena_query_geom       1,102,436       858,982                -243,454  -22.08%   x 1.28
 bench_arena_query_norm       589,876         456,101                -133,775  -22.68%   x 1.29
 bench_arena_query_norm_geom  1,098,293       869,965                -228,328  -20.79%   x 1.26
 bench_query                  592,841         432,123                -160,718  -27.11%   x 1.37

clbarnes / nblast-rs

Use single nearest neighbor method instead of iterator #21