Remove arrow-ord dependency in arrow-cast due to RunEndEncoded casting

#8589 introduced a dependency on arrow-ord in arrow-cast because it uses the `partition` kernel for casting RunEndEncoded (REE) arrays. We are trying to keep the number of dependencies to a minimum.

> Probably we can also partition a (single) column using `eq` and an offset to look for consecutive rows which are different.

> Something like

```rust
let arr = ...;
let arr_shift1 = arr.slice(1, arr.len()-1);
let transitions = eq(arr, arr_shift_1);
```

> However, the `eq` kernels is also in `arrow-ord` so I am not sure there is a way around it

_Originally posted by @alamb in https://github.com/apache/arrow-rs/pull/8589#discussion_r2456004251_