Add support for Utf8Scalar

[emilk]

Closes #105

[codecov-commenter]

Codecov Report

Merging #106 (39eb555) into main (b2fd8f8) will increase coverage by 0.08%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   94.63%   94.71%   +0.08%     
==========================================
  Files           8        8              
  Lines        1565     1589      +24     
==========================================
+ Hits         1481     1505      +24     
  Misses         84       84              

Impacted Files	Coverage Δ
arrow2_convert/src/deserialize.rs	100.00% <100.00%> (ø)
arrow2_convert/src/field.rs	100.00% <100.00%> (ø)
arrow2_convert/src/serialize.rs	100.00% <100.00%> (ø)

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[ncpenke]

The change looks overall. One question, so that I understand and can document the use-case of this better, when is it preferable to use Utf8Scalar vs String?

[emilk]

I was about to write that Utf8Scalar has O(1) cloning and slicing, but reading the docs now that isn't the case at all 😬

I assumed it did because Buffer does, but I was a bit hasty in my assumptions. I'll need to rethink this a bit 😅