get_value_box duplication
`tslib.get_value_box` seems out of place.  The only place it is used is in `base.index`.  Meanwhile `_libs.index.get_value_at` looks very similar but is never used.  (It also looks similar to a chunk of code in `IndexEngine.get_value`

Is it possible that `index.get_value_at` was intended to replace `tslib.get_value_box`?