Fixed casting dictionary keys

[ritchie46]

Updating arrow2 had some failed test in polars. It turned out that casting dictionary keys was a no-op. This PR fixes that and adds a test.

[codecov]

Codecov Report

Merging #1143 (a59396b) into main (98e4913) will increase coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1143      +/-   ##
==========================================
+ Coverage   83.59%   83.63%   +0.03%     
==========================================
  Files         366      366              
  Lines       35901    35912      +11     
==========================================
+ Hits        30013    30035      +22     
+ Misses       5888     5877      -11     

Impacted Files	Coverage Δ
src/compute/cast/dictionary_to.rs	26.49% <50.00%> (+4.08%)	⬆️
src/chunk.rs	83.33% <0.00%> (-7.15%)	⬇️
src/io/ipc/read/reader.rs	95.98% <0.00%> (-0.67%)	⬇️
src/io/ipc/read/schema.rs	94.56% <0.00%> (-0.32%)	⬇️
src/array/growable/utf8.rs	95.89% <0.00%> (-0.06%)	⬇️
src/io/csv/utils.rs	85.29% <0.00%> (ø)
src/array/growable/list.rs	97.14% <0.00%> (ø)
src/compute/regex_match.rs	82.35% <0.00%> (ø)
src/io/avro/read/header.rs	100.00% <0.00%> (ø)
src/io/avro/write/header.rs	100.00% <0.00%> (ø)
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 98e4913...a59396b. Read the comment docs.

[jorgecarleitao]

Looks awesome, thanks a lot, @ritchie46 !

Left minor suggestions to guide readability of unsafe