growable::union is broken for dense unions

[jleibs]

The derived array ends up repeating values from the first union in every extension.

Minimal unit test demonstrating the problem here: https://github.com/rerun-io/arrow2/blob/b082fd9b9bdd2a54ed68a6dc5570e457e6c24b00/tests/it/array/growable/union.rs#L73

---- array::growable::union::complex_dense stdout ----
thread 'array::growable::union::complex_dense' panicked at 'assertion failed: `(left == right)`
  left: `UnionArray[1, 2, c, d, e, 6]`,
 right: `UnionArray[1, 2, c, c, d, 1]`', tests/it/array/growable/union.rs:116:5

The crux of the problem occurs here:

• arrow2/src/array/growable/union.rs

  Line 76 in 211be21

  x.extend(offsets);

The new offsets that are being extended here refer to positions in the original values-array, but need to account for the fact that these will be in a newly-extended values array and need to be adjusted to reflect their new position accordingly.

I'll put up a PR to resolve this in the next day or two, but wanted an issue to refer to when I submit that PR.