Added support to round-trip dictionary arrays on parquet

[jorgecarleitao]

Closes #211. Dictionary-encoding is a popular and important technique to reduce memory usage.

Parquet has an encoding specifically for this (PlainDictionary and RleDictionary). Arrow has "DictionaryArray" specifically for this. This PR bridges these two by allowing arrow2 to read and write dictionary arrays to and from parquet.

The semantics is as follows:

When writing to parquet, it is now possible to use Encoding::PlainDictionary and Encoding::RleDictionary when writing DataType::Dictionary fields. This causes the array to be dictionary-encoded into parquet (two pages: one with values one with indices).

When reading from parquet, if the arrow schema on the metadata contains a field with DataType::Dictionary and the pages are dictionary-encoded, we read them to a DictionaryArray (all other cases error with not yet implemented).

As before, dictionary-encoded pages without an explicit DataType::Dictionary in the schema are read to their "natural", non-dictionary, DataType.

[codecov]

Codecov Report

Merging #232 (063ede8) into main (fa3c2ce) will decrease coverage by 0.05%.
The diff coverage is 66.40%.

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
- Coverage   76.81%   76.76%   -0.06%     
==========================================
  Files         229      231       +2     
  Lines       19617    19768     +151     
==========================================
+ Hits        15068    15174     +106     
- Misses       4549     4594      +45     

Impacted Files	Coverage Δ
src/io/parquet/write/binary/nested.rs	95.65% <ø> (ø)
src/io/parquet/write/boolean/basic.rs	97.56% <ø> (ø)
src/io/parquet/write/boolean/nested.rs	95.65% <ø> (ø)
src/io/parquet/write/fixed_len_bytes.rs	88.46% <ø> (ø)
src/io/parquet/write/primitive/nested.rs	95.65% <ø> (ø)
src/io/parquet/write/utf8/nested.rs	95.65% <ø> (ø)
src/io/parquet/write/dictionary.rs	43.24% <43.24%> (ø)
src/io/parquet/read/fixed_size_binary.rs	43.00% <50.00%> (ø)
src/io/parquet/write/mod.rs	68.56% <58.62%> (-1.35%)	⬇️
src/io/parquet/read/primitive/dictionary.rs	70.14% <70.14%> (ø)
... and 31 more

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