use ArrayOfArrays for return value to reduce the number of allocated arrays

[KristofferC]

A long standing gripe for me has been that indices and distances are returned as standard nested Arrays. Typically, each inner array hold quite a small number of neighbors so it means that we allocate a large number of small arrays.

Using ArrayOfArrays, these are stored contigously in one large flat array instead.

The difference in allocations can be readily seen:

julia> input = rand(3, 10^6);

julia> tree = KDTree(rand(3, 10^6));

julia> @time knn(tree, input, 5);
  1.538003 seconds (2.00 M allocations: 221.253 MiB, 10.03% gc time)

julia> @time knn(tree, input, 5);
  1.489310 seconds (98 allocations: 189.884 MiB, 0.29% gc time)

[KristofferC]

One potential issue with this is that getindex is slower on AoA so depending on how people use the return value they might encounter performance regressions.