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ARROW-6690: [Rust] [DataFusion] Optimize aggregates without GROUP BY to use SIMD

45% improvement of benchmark for aggregate operations that do not have a GROUP BY. This is an optimization that we already had in the original query execution logic and I have just ported that over to the new physical query plan.

```
aggregate_query_no_group_by
                        time:   [13.228 us 13.246 us 13.264 us]
                        change: [-45.563% -45.218% -44.879%] (p = 0.00 < 0.05)
                        Performance has improved.
```

Closes #5606 from andygrove/ARROW-6690 and squashes the following commits:

3d7db2509 <Andy Grove> code cleanup
0338f5689 <Andy Grove> Restructure code to avoid standalone functions
539956dc0 <Andy Grove> Use SIMD for aggregate queries without GROUP BY clause

Authored-by: Andy Grove <andygrove73@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>