Made DynComparator Send+Sync

[yjshen]

This PR is a port of apache/arrow-rs#542 . As I quote the rationale here:

The cost of building a comparator (initialising a DynComparator) is often significantly higher than the actual cost of executing the comparator's closure on two row IDs. Therefore it makes sense to build the comparator once, and re-use the returned DynComparator for each row you are comparing the two arrays on.

Due to the explicit lifetime of DynComparator I recently found it problematic to store the DynComparator on an object that was used across threads in an async environment.

By refactoring the build_compare implementations I was able to remove the lifetime and it's much easier to use in the Datafusion operator I'm improving.

[codecov]

Codecov Report

Merging #414 (6d8c76c) into main (16a09ee) will decrease coverage by 0.00%.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##             main     #414      +/-   ##
==========================================
- Coverage   80.96%   80.95%   -0.01%     
==========================================
  Files         347      347              
  Lines       22141    22135       -6     
==========================================
- Hits        17926    17920       -6     
  Misses       4215     4215              

Impacted Files	Coverage Δ
src/array/ord.rs	64.21% <86.95%> (-2.13%)	⬇️

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[yjshen]

Thank you @jorgecarleitao for the quick response. 👍