Sealed 2 traits to forbid downstream implementations

[jorgecarleitao]

Also removed the unsafe from them, since they are now sealed (since it was just a way to avoid others from implementing).

[codecov]

Codecov Report

Merging #621 (6001c9d) into main (b3ed162) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #621      +/-   ##
==========================================
+ Coverage   69.70%   69.72%   +0.01%     
==========================================
  Files         299      299              
  Lines       16599    16599              
==========================================
+ Hits        11571    11573       +2     
+ Misses       5028     5026       -2     

Impacted Files	Coverage Δ
src/array/specification.rs	73.17% <ø> (ø)
src/io/parquet/write/levels.rs	94.68% <ø> (ø)
src/types/mod.rs	23.61% <ø> (ø)
src/io/parquet/read/nested_utils.rs	78.43% <0.00%> (+0.98%)	⬆️
src/compute/arithmetics/time.rs	46.93% <0.00%> (+2.04%)	⬆️

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