Change signature of PrimitiveScalar::value to return reference

[ncpenke]

The use-case is to be able to write generic code that uses scalars with Array iterators (for example in arrow2-convert for the deserialization path). This is much easier if the PrimitiveScalar::value() is consistent with the rest of scalars and returns a reference.

This could be too high risk of a change if too many downstream users will be affected, so alternately a new method PrimitiveScalar::value_ref() could be introduces that returns the reference. But that seemed less clean, so opted for this change to get feedback. There seems to be good test coverage around scalars, and all the tests are passing.

[codecov]

Codecov Report

Merging #1129 (b8759df) into main (81ab424) will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #1129      +/-   ##
==========================================
- Coverage   83.37%   83.37%   -0.01%     
==========================================
  Files         367      367              
  Lines       35574    35574              
==========================================
- Hits        29661    29659       -2     
- Misses       5913     5915       +2     

Impacted Files	Coverage Δ
src/compute/arithmetics/decimal/mul.rs	75.77% <0.00%> (ø)
src/compute/arithmetics/time.rs	72.19% <50.00%> (ø)
src/compute/arithmetics/decimal/div.rs	95.39% <100.00%> (ø)
src/compute/arithmetics/mod.rs	74.10% <100.00%> (ø)
src/scalar/primitive.rs	80.00% <100.00%> (ø)
src/array/binary/mod.rs	88.88% <0.00%> (-1.24%)	⬇️
src/io/ipc/read/reader.rs	95.66% <0.00%> (-0.37%)	⬇️
src/bitmap/immutable.rs	81.48% <0.00%> (+0.61%)	⬆️
src/bitmap/utils/slice_iterator.rs	98.78% <0.00%> (+1.21%)	⬆️

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