Make merge_sort_slices MergeSortSlices public

[sundy-li]

In OLAP database system, sort/merge_sort is not just for one column.

The unit of sort compute is block, we should make merge_sort_slices public, then we can take the indices from the block to build new blocks.

[codecov]

Codecov Report

Merging #243 (ea2f82d) into main (eea4d33) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #243   +/-   ##
=======================================
  Coverage   76.81%   76.81%           
=======================================
  Files         229      229           
  Lines       19617    19617           
=======================================
  Hits        15068    15068           
  Misses       4549     4549           

Impacted Files	Coverage Δ
src/compute/merge_sort/mod.rs	94.09% <100.00%> (ø)

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[sundy-li]

We also need materialize_merge_indices to reuse the iterator to sort other arrays.

Something like:

https://github.com/datafuselabs/datafuse/blob/8dbd277f90a7251806d90ccbe002b51954c63526/common/datablocks/src/kernels/data_block_sort.rs#L135-L161