fix: bug in offset calculation for unions

[helgikrs]

Which issue does this PR close?

Closes #862.

What changes are included in this PR?

The value_offset function only read the least significant byte in the
offset array, causing issues with unions with more than 255 rows of any
given variant. This fixes the issue by reading the entire i32 offset.

[alamb]

Thank you @helgikrs

Note there is outstanding work to update the Rust union implementation to a newer version of the arrow spec -- see #85 and #814 if you are interested

I ran the test without the changes in this PR and it fails thusly 👍 so in my mind the PR is good to go.

---- array::array_union::tests::test_dense_i32_large stdout ----
thread 'array::array_union::tests::test_dense_i32_large' panicked at 'assertion failed: `(left == right)`
  left: `0`,
 right: `256`', arrow/src/array/array_union.rs:366:13
stack backtrace:
   0: rust_begin_unwind
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/std/src/panicking.rs:517:5
   1: core::panicking::panic_fmt
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/core/src/panicking.rs:101:14
   2: core::panicking::assert_failed_inner
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/core/src/panicking.rs:183:17
   3: core::panicking::assert_failed
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/core/src/panicking.rs:140:5
   4: arrow::array::array_union::tests::test_dense_i32_large
             at ./src/array/array_union.rs:366:13
   5: arrow::array::array_union::tests::test_dense_i32_large::{{closure}}
             at ./src/array/array_union.rs:338:5
   6: core::ops::function::FnOnce::call_once
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/core/src/ops/function.rs:227:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/09c42c45858d5f3aedfa670698275303a3d19afa/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

cc @nevi-me @paddyhoran

[codecov-commenter]

Codecov Report

Merging #863 (3b4a836) into master (2662bd8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 3b4a836 differs from pull request most recent head 43900cd. Consider uploading reports for the commit 43900cd to get more accurate results

@@           Coverage Diff           @@
##           master     #863   +/-   ##
=======================================
  Coverage   82.66%   82.67%           
=======================================
  Files         168      168           
  Lines       48172    48197   +25     
=======================================
+ Hits        39823    39848   +25     
  Misses       8349     8349           

Impacted Files	Coverage Δ
arrow/src/array/array_union.rs	89.88% <100.00%> (+0.61%)	⬆️

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[paddyhoran]

Thank you @helgikrs, LGTM.