`lexsort_to_indices_impl` ignores `build_compare_fn` for single column

[tijsvd]

Hi,

I was playing with arrow2::compute::lexsort, and noticed in the source that build_compare_fn is not used initially if there is only a single sort column. It is used if sort_to_indices fails (unsupported type).

This is sort-of documented, as in the function documentation hints that it should be used in case of unsupported sort types (but it doesn't say only for this case). I would expect to be able to use this function for custom orderings in general, for example. In my case, I won't support null keys, and I found that a custom compare fn that doesn't check nulls performs measurably better.

If the single-column optimization is useful (I guess so as the implementation of sort_to_indices is fully generated for each type), then perhaps it should be done in lexsort_to_indices instead of lexsort_to_indices_impl?