migrate partition kernel to use Iterator trait

[jimexist]

It should be possible to write the below as an iterator, thereby avoiding the allocation of the vector.

The general idea: create a struct with a method try_new. That method initializes the struct's content and errors if something is wrong (e.g. empty columns). It also initializes lexicographical_comparator.

Next, implement Iterator<Item=usize> for that struct where Iterator::next yields previous_partition_point and increments it as written in this function.

Originally posted by @jorgecarleitao in #424 (comment)