Improved bitpacking

[jorgecarleitao]

This PR ports apache/arrow-rs#2278 to parquet2. Credit to the design and implementation of the unpacking path go to @tustvold - it is 5-10% faster than the bitpacking crate 🚀

Additionally, it adds the corresponding packing code path, thereby completely replacing the dependency on bitpacking.

It also adds some traits that allows code to be written via generics.

A curious observation is that, with this PR, parquet2 no longer executes unsafe code (bitpacking had some) 🎉

Backward changes:

• renamed parquet2::encoding::bitpacking to parquet2::encoding::bitpacked
• parquet2::encoding::bitpacked::Decoder now has a generic parameter (output type)
• parquet2::encoding::bitpacked::Decoder::new's second parameter is now a usize

[codecov-commenter]

Codecov Report

Merging #176 (1f597af) into main (d1c012c) will increase coverage by 0.03%.
The diff coverage is 89.81%.

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   85.29%   85.32%   +0.03%     
==========================================
  Files          78       82       +4     
  Lines        7916     8110     +194     
==========================================
+ Hits         6752     6920     +168     
- Misses       1164     1190      +26     

Impacted Files	Coverage Δ
src/encoding/mod.rs	100.00% <ø> (ø)
src/write/file.rs	92.89% <ø> (-0.05%)	⬇️
src/encoding/bitpacked/mod.rs	71.27% <71.27%> (ø)
src/encoding/bitpacked/unpack.rs	88.57% <88.57%> (ø)
src/encoding/bitpacked/pack.rs	89.39% <89.39%> (ø)
src/encoding/bitpacked/encode.rs	96.77% <96.77%> (ø)
src/encoding/bitpacked/decode.rs	98.47% <98.47%> (ø)
src/encoding/delta_bitpacked/decoder.rs	99.22% <100.00%> (ø)
src/encoding/delta_bitpacked/encoder.rs	100.00% <100.00%> (ø)
src/encoding/delta_bitpacked/mod.rs	100.00% <100.00%> (+27.69%)	⬆️
... and 5 more

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[jorgecarleitao]

@tustvold, you may wish to port the packing part to parquet.

If it would be ok for you, the packing and unpacking functions that we wrote could live in a separate crate. I think that they are well encapsulated and folks that want this encoding somewhere could benefit from it. I for once would like to use them in https://github.com/DataEngineeringLabs/orc-format, since ORC also has bitpacked runs.