feature gate csv functionality

[ritchie46]

Which issue does this PR close?

This PR puts the csv parsing logic behind a feature gate. Thereby removing csv crate as a dependency. This closes #309.

I also added the csv feature to the default features to reduce the probability of unexpected changes for users that relied on the default features.

[alamb]

Thank you @ritchie46 !

I wonder if we should test this in CI so that it doesn't break? When I ran the tests it seems as if one of the doc tests fails without this feature

cargo test -p arrow --no-default-features

---- src/util/string_writer.rs - util::string_writer (line 25) stdout ----
error[E0432]: unresolved import `arrow::csv`
 --> src/util/string_writer.rs:27:5
  |
5 | use arrow::csv;
  |     ^^^^^^^^^^ no `csv` in the root

error: aborting due to previous error

For more information about this error, try `rustc --explain E0432`.
Couldn't compile the test.

failures:
    src/util/string_writer.rs - util::string_writer (line 25)

test result: FAILED. 87 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 6.20s

error: test failed, to rerun pass '--doc'
(arrow_dev) alamb@ip-10-0-0-124:~/Software/arrow-rs$ 

[ritchie46]

I wonder if we should test this in CI so that it doesn't break? When I ran the tests it seems as if one of the doc tests fails without this feature

Oh, I missed that one. Will fix that and I will see if I can add a CI change.

[codecov-commenter]

Codecov Report

Merging #312 (a741a99) into master (c863a2c) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #312      +/-   ##
==========================================
- Coverage   82.52%   82.52%   -0.01%     
==========================================
  Files         162      162              
  Lines       44007    44007              
==========================================
- Hits        36316    36315       -1     
- Misses       7691     7692       +1     

Impacted Files	Coverage Δ
arrow/src/csv/reader.rs	88.36% <ø> (ø)
arrow/src/csv/writer.rs	82.64% <ø> (ø)
arrow/src/error.rs	8.88% <ø> (ø)
arrow/src/util/string_writer.rs	73.33% <ø> (ø)
parquet/src/encodings/encoding.rs	94.85% <0.00%> (-0.20%)	⬇️

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[alamb]

I love it -- this looks great. Thanks @ritchie46

Thoughts @andygrove / @jorgecarleitao / @nevi-me ?