arrow

ARROW-11488: [Rust] Don't leak memory in StructBuilder

This PR fixes the following MIRI error (example from  https://github.com/apache/arrow/pull/9373/checks?check_run_id=1830406627):

```
test array::builder::tests::test_string_dictionary_builder_with_reserved_null_value ... ok
error: Undefined Behavior: trying to reborrow for SharedReadWrite at alloc9600855, but parent tag <26451274> does not have an appropriate item in the borrow stack
    --> arrow/src/array/builder.rs:1434:9
     |
1434 |         self.field_anys[i].downcast_mut::<T>()
     |         ^^^^^^^^^^^^^^^^^^ trying to reborrow for SharedReadWrite at alloc9600855, but parent tag <26451274> does not have an appropriate item in the borrow stack
     |
     = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
     = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

     = note: inside `array::builder::StructBuilder::field_builder::<array::builder::GenericStringBuilder<i32>>` at arrow/src/array/builder.rs:1434:9
note: inside `array::builder::tests::test_struct_array_builder` at arrow/src/array/builder.rs:2824:30
    --> arrow/src/array/builder.rs:2824:30
     |
2824 |           let string_builder = builder
     |  ______________________________^
2825 | |             .field_builder::<StringBuilder>(0)
     | |______________________________________________^
note: inside closure at arrow/src/array/builder.rs:2810:5
    --> arrow/src/array/builder.rs:2810:5
     |
2810 | /     fn test_struct_array_builder() {
2811 | |         let string_builder = StringBuilder::new(4);
2812 | |         let int_builder = Int32Builder::new(4);
2813 | |
...    |
2889 | |         }
2890 | |     }
     | |_____^
     = note: inside `<[closure@arrow/src/array/builder.rs:2810:5: 2890:6] as
```

Closes #9409 from sfackler/struct-memory-leak

Authored-by: Steven Fackler <sfackler@palantir.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>