lexicographical_partition_ranges does not need to materialize indices

[jhorstmann]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

the lexicographical_partition_ranges function currently materializes a vector of incrementing indices, which it then uses to call slice::partition_point. The benefit is that we can directly reuse a standard rust function, the downside is that the vector of indices can be quite large and that it adds another indirection. I think by duplicating the functionality of partition_points to work with a range we could avoid this allocation and indirection.

Describe the solution you'd like
Reimplement partition_points to work with a range. The implementation in the standard library calls a relatively simple binary search which could be simplified for this usecase. The predicate could be used directly so there is no need for using the Ordering enum while searching.