Added iterator for StructArray

[illumination-k]

I implemented Iterator for StructArray.
Also, the struct_.rs file was changed to a directory with mod.rs and the ffi part was split into ffi.rs.

If you need me to open an issue on this PR, I will do it.

[codecov]

Codecov Report

Merging #613 (ba1ca05) into main (9d4107c) will decrease coverage by 0.03%.
The diff coverage is 67.74%.

@@            Coverage Diff             @@
##             main     #613      +/-   ##
==========================================
- Coverage   79.61%   79.58%   -0.04%     
==========================================
  Files         403      406       +3     
  Lines       24838    24882      +44     
==========================================
+ Hits        19775    19802      +27     
- Misses       5063     5080      +17     

Impacted Files	Coverage Δ
src/array/struct_/mod.rs	56.92% <ø> (ø)
tests/it/array/mod.rs	100.00% <ø> (ø)
src/array/struct_/iterator.rs	40.00% <40.00%> (ø)
src/array/struct_/ffi.rs	88.88% <88.88%> (ø)
tests/it/array/struct_/iterator.rs	100.00% <100.00%> (ø)
src/compute/arithmetics/time.rs	46.93% <0.00%> (+2.04%)	⬆️

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[jorgecarleitao]

Thanks for the PR! No need for an issue :)

The iterators of arrays are iterators over rows.

An iterator over a StructArray would be something like Iterator<Item=Option<Vec<Box<dyn Scalar>>>, since each row is composed by multiple entries (one per field) where the Option is the Struct's validity.

[illumination-k]

Thanks for your comments! I understood it wrong. Maybe, I think I have corrected it.

[jorgecarleitao]

Thanks a lot, @illumination-k, Looks great!

I left a comment to simplify the loop.

The idea in Rust is that when using iterator pattern, rust skips bound checks (it also makes it a bit easier to read for non-C developers)