Delay deserialization of dictionary pages

[jorgecarleitao]

This crate:

• assumes a specific in-memory format for (decoded) dictionary pages
• decompresses and deserializes the dictionary pages during reading

These are problematic because:

• in-memory formats can't deserialize parquet's dictionary pages directly to their own format, having to perform &[u8] -> this crate DictPage -> their format. Because this crate DictPage -> their format may depend on logical types, this has to be done on every array (instead of once per dictionary page).
• reading pages is IO-bounded, but deserializing the dictionary page is CPU-bounded. In async, this mix causes the reader to be blocked by this (potentially very expensive) operation

This PR

This PR modifies the page readers to output the enum Page instead of a DataPage, no longer attaching a decoded dictionary page (Arc<dyn DictPage>) to every page. Users must now handle the deserialization of the dictionary page themselves by matching the enum variants. This allows them to

• deserialize the dict page to their target in-memory format, thus replacing &[u8] -> this crate DictPage -> their in-memory format by &[u8] -> their in-memory format
• no longer have blocking CPU-bounded on their async context.

The change to the example and integration tests shows the required migration

[codecov-commenter]

Codecov Report

Merging #160 (3079ef8) into main (82b1115) will increase coverage by 0.34%.
The diff coverage is 76.53%.

@@            Coverage Diff             @@
##             main     #160      +/-   ##
==========================================
+ Coverage   73.55%   73.90%   +0.34%     
==========================================
  Files          78       74       -4     
  Lines        3778     3652     -126     
==========================================
- Hits         2779     2699      -80     
+ Misses        999      953      -46     

Impacted Files	Coverage Δ
src/compression.rs	84.78% <ø> (ø)
src/read/mod.rs	82.14% <ø> (ø)
src/read/page/stream.rs	61.70% <0.00%> (-0.12%)	⬇️
src/write/compression.rs	58.33% <0.00%> (-2.33%)	⬇️
src/write/page.rs	75.58% <ø> (ø)
src/deserialize/boolean.rs	60.00% <33.33%> (ø)
src/read/page/indexed_reader.rs	62.12% <47.05%> (+6.71%)	⬆️
src/page/mod.rs	72.72% <91.66%> (+2.56%)	⬆️
src/deserialize/binary.rs	70.37% <100.00%> (-2.05%)	⬇️
src/deserialize/fixed_len.rs	69.23% <100.00%> (-1.51%)	⬇️
... and 7 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 82b1115...3079ef8. Read the comment docs.