Added example to read parquet in parallel with rayon

[jorgecarleitao]

This shaves a lot of time when reading many columns. The tradeoff, as usual, is that it loads all pages of a given row group into memory, to be parallelized via par_iter.

There is a middle ground here were we can use Rayon's par_bridge, but at that point we are mixing IO and CPU-bounded tasks, which for network IO / s3 makes things more difficult.

[jorgecarleitao]

cc @ritchie46 , I think this is how polars can do it. ^_^

[ritchie46]

Awesome! Thanks for this! :)

[codecov]

Codecov Report

Merging #658 (76c96ed) into main (299818a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #658   +/-   ##
=======================================
  Coverage   69.55%   69.56%           
=======================================
  Files         299      299           
  Lines       16735    16735           
=======================================
+ Hits        11640    11641    +1     
+ Misses       5095     5094    -1     

Impacted Files	Coverage Δ
src/bitmap/utils/slice_iterator.rs	92.53% <0.00%> (+1.49%)	⬆️

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