Made substring kernel on utf8 take chars into account.

[ritchie46]

The substring kernel, sliced [u8] data, not taking account utf8 char boundaries. This PR fixes that.

[codecov]

Codecov Report

Merging #568 (4851f34) into main (ed8836f) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##             main     #568      +/-   ##
==========================================
- Coverage   78.90%   78.88%   -0.02%     
==========================================
  Files         395      395              
  Lines       24542    24680     +138     
==========================================
+ Hits        19364    19470     +106     
- Misses       5178     5210      +32     

Impacted Files	Coverage Δ
src/compute/substring.rs	91.66% <80.00%> (-5.60%)	⬇️
tests/it/compute/substring.rs	89.35% <80.00%> (-0.23%)	⬇️
src/io/ipc/write/serialize.rs	64.08% <0.00%> (-16.30%)	⬇️
src/io/ipc/compression.rs	84.21% <0.00%> (-15.79%)	⬇️
src/compute/cast/dictionary_to.rs	25.00% <0.00%> (-2.03%)	⬇️
src/io/ipc/read/read_basic.rs	73.95% <0.00%> (-1.05%)	⬇️
tests/it/io/ipc/write/file.rs	100.00% <0.00%> (ø)
src/io/ipc/read/array/boolean.rs	50.00% <0.00%> (ø)
src/io/ipc/read/array/dictionary.rs	50.00% <0.00%> (ø)
tests/it/compute/cast.rs	99.35% <0.00%> (+0.05%)	⬆️
... and 8 more

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[jorgecarleitao]

Thanks! PRed a proposal to your PR to remove unsafe :P

[ritchie46]

Thanks! PRed a proposal to your PR to remove unsafe :P

Way better! I've left some questions.

[jorgecarleitao]

CI is unrelated and being fixed by apache/arrow#11609.