Add tracing option for reading strings as (Large)Utf8

[jkylling]

Should be on top of #245

Is there something missing in coerce_primitive_type? There seems to be an asymmetry between Utf8 and LargeUtf8 there.

Could some instructions on how to run tests be added to the README. To run the full test suite the appropriate --features and --cfg flags must be set, and I was a bit confused about which flags to set to make the tests run.

[chmp]

I would prefer to keep both changes, large lists and Utf8, in a single PR as they are so closely related. Probably this one.

Re how to execute tests the x.py script bundles all common commands and is documented in Development.md. It could be advertised a bit more prominently, though. Running python x.py test will set everything up.

Re asymetry: as only LargeUtf8 can be encountered during from_samples, only this case is handled in the coercion.

[chmp]

Thanks a lot. Looks great, but you're right that coerce_primitives should be updated.

Please also expand the coercion tests. I will look them up for you.

[chmp]

I would prefer to keep both changes, large lists and Utf8, in a single PR as they are so closely related. Probably this one.

Okay. Actually the tests for the coercion could be a bit more tricky. Separate PRs is the way to go :)

[chmp]

As for the tests, I would include the coercion tests here:

E.g., by changing the macro to:

Potential test impl

    macro_rules! test {
        ($name:ident, $($data:tt)*) => {
            mod $name {
                use super::super::*;
                
                #[test]
                fn large_utf8() -> PanicOnError<()> {
                    let expected = SerdeArrowSchema::from_value(&json!([
                       {
                          "name": "date",
                          "data_type": "LargeUtf8",
                          "nullable": true,
                      },
                  ]))?;

                  let data = json!($($data)*);
                  let actual = SerdeArrowSchema::from_samples(&data, TracingOptions::default().guess_dates(true))?;
                  assert_eq!(actual, expected);
                  Ok(())
              }
              #[test]
                fn utf8() -> PanicOnError<()> {
                    let expected = SerdeArrowSchema::from_value(&json!([
                       {
                          "name": "date",
                          "data_type": "Utf8",
                          "nullable": true,
                      },
                  ]))?;

                  let data = json!($($data)*);
                  let actual = SerdeArrowSchema::from_samples(&data, TracingOptions::default().guess_dates(true).strings_as_large_utf8(false))?;
                  assert_eq!(actual, expected);
                  Ok(())
              }
            }
        };
    }

[chmp]

@jkylling Thanks a lot for your changes!

I took the opportunity to slightly touch up the TracingOptions docs and make it more consistent between the different items.