Simplified API to write files

[jorgecarleitao]

This simplifies the API to write a file by giving the user more control over when to emit a Row group to write.

Thanks to /u/dexterduck for the feedback on reddit that led to this PR!

[codecov-commenter]

Codecov Report

Merging #78 (e24554b) into main (2194ee4) will decrease coverage by 1.54%.
The diff coverage is 2.35%.

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
- Coverage   69.30%   67.76%   -1.55%     
==========================================
  Files          66       68       +2     
  Lines        3691     3800     +109     
==========================================
+ Hits         2558     2575      +17     
- Misses       1133     1225      +92     

Impacted Files	Coverage Δ
integration-tests/src/lib.rs	0.00% <0.00%> (ø)
src/write/file.rs	45.16% <0.00%> (-54.84%)	⬇️
src/write/stream.rs	0.00% <0.00%> (ø)
src/read/mod.rs	93.72% <100.00%> (-0.06%)	⬇️
src/write/mod.rs	92.40% <100.00%> (-1.89%)	⬇️
src/encoding/hybrid_rle/mod.rs	86.36% <0.00%> (-1.57%)	⬇️
src/lib.rs	78.62% <0.00%> (-1.54%)	⬇️
src/encoding/hybrid_rle/decoder.rs	94.11% <0.00%> (+1.96%)	⬆️

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[sydduckworth]

Looks good to me!

Would be convenient to have an interface for writing Parquet files that implements Sink but I'd be happy to put in an issue for that since it's likely out of scope of this PR.

[jorgecarleitao]

Thanks @dexterduck ! You mean futures::Sink?

[sydduckworth]

@jorgecarleitao yep! It's a relatively minor thing but it would be nice to have an interface that implements futures::Sink<RowGroupIter<'_, _>> where closing the sink writes the Parquet footer and then closes the underlying writer.
Kind of provides the best of both worlds since you can write row by row if needed, but you can also use futures::StreamExt::forward to fully write out a stream of row groups to a file in a single call.

[jorgecarleitao]

Thanks for your review, @dexterduck and suggestion. I agree. Would you be able to work on the Sink? I am not familiar with that API. It would be easier for someone that is familiar with it.

[sydduckworth]

Sure, I'll give it a shot and put a PR in!