Implement Sync for ArrayParts

[gaumut]

Closes #1548

[gaumut]

Switching the Sync impl from ArrayParts to ArrayRef should allow this to pass the tests.

[nilgoyette]

@akern40 ArrayRef is your work. You're the best person around to know if we actually need this change.

I'm really surprised that this bug didn't appear during your refactor. We have plenty of parallel code and tests in this repo.

[gaumut]

I’m not that surprised that it didn’t show up because while it doesn’t prevent this

fn test1(a: &ArrayRef1<f64>) { // works
    Zip::from(a).par_for_each(|v| {
        let _ = v;
    });
}

it does prevent doing this (which is much less common)

fn test2(a: &ArrayRef1<f64>) { // does not compile
    let b = array![0.];
    Zip::from(&b).par_for_each(|_| {
        let _ = a.len();
    });
}

My rational is that since both of those examples work with ArrayView (I guess because ArrayView inherit the Sync impl from ArrayBase) this should also work with ArrayRef.

[gaumut]

Also I saw that Send is implemented on ArrayBase, if this PR is accepted maybe Send should also be implemented on ArrayRef.