Simplified dictionary indexes

[jorgecarleitao]

The Box<DataType> in DataType::Dictionary(Box<DataType>, _) is not sufficiently restrictive as there are only 8 possible variants on the dictionary keys. This PR changes it to DataType::Dictionary(IntegerType, _), allowing match to cover all cases.

Backward incompatible changes

As a user declaring dictionaries, to migrate to the new behavior, change

DataType::Dictionary(Box::new(DataType::Int32), Box::new(values))

to

DataType::Dictionary(i32::KEY_TYPE, Box::new(values))

As a user writing kernels, use arrow2::datatypes::IntegerType to match the cases.

[codecov]

Codecov Report

Merging #584 (7cc30b6) into main (f146097) will increase coverage by 0.05%.
The diff coverage is 56.86%.

@@            Coverage Diff             @@
##             main     #584      +/-   ##
==========================================
+ Coverage   78.99%   79.05%   +0.05%     
==========================================
  Files         399      399              
  Lines       24752    24736      -16     
==========================================
+ Hits        19554    19555       +1     
+ Misses       5198     5181      -17     

Impacted Files	Coverage Δ
src/array/growable/mod.rs	21.56% <0.00%> (-4.63%)	⬇️
src/compute/aggregate/memory.rs	20.51% <0.00%> (ø)
src/compute/cast/dictionary_to.rs	24.39% <0.00%> (-0.61%)	⬇️
src/io/avro/read/nested.rs	44.44% <ø> (ø)
src/io/json_integration/mod.rs	0.00% <ø> (ø)
src/types/mod.rs	23.61% <ø> (ø)
src/array/ord.rs	71.25% <22.22%> (ø)
src/array/display.rs	50.37% <25.00%> (+<0.01%)	⬆️
src/array/mod.rs	59.64% <25.00%> (ø)
src/io/csv/write/serialize.rs	33.93% <33.33%> (ø)
... and 29 more

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