Implement UnionArray FieldData using Type Erasure

[tustvold]

Which issue does this PR close?

Follow on from #1591

Rationale for this change

mutable_buffer_to_builder created an untyped back-door to creating a BufferBuilder<T> with arbitrary MutableArray. This makes it hard to reason about the invariants of the MutableArray belonging to a BufferBuilder.

What changes are included in this PR?

This instead implements UnionArray using type-erasure, which is safer and less code.

Are there any user-facing changes?

No

[codecov-commenter]

Codecov Report

Merging #1842 (9b16115) into master (c08e532) will increase coverage by 0.02%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #1842      +/-   ##
==========================================
+ Coverage   83.49%   83.52%   +0.02%     
==========================================
  Files         201      201              
  Lines       56903    56871      -32     
==========================================
- Hits        47511    47499      -12     
+ Misses       9392     9372      -20     

Impacted Files	Coverage Δ
arrow/src/buffer/mutable.rs	90.53% <ø> (ø)
arrow/src/array/builder.rs	87.75% <87.50%> (+0.87%)	⬆️
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (+0.22%)	⬆️

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[alamb]

Looks like a nice cleanup to me.

I also checked and this does not appear to change any of the public API, which is 👍