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ARROW-9679: [Rust] [DataFusion] More efficient creation of final batch from HashAggregateExec

Instead of walking through the map containing accumulators multiple times (once per grouping expression and once per aggregate expression) let's just walk through it once! Safer and faster.

Other changes in this PR:

- min and max had the wrong data type. MIN(expr) and MAX(expr) should always have the same data type as the underlying expression. They cannot overflow.
- I shortened the name of one of the existing macros so that match statements would fit on a single line.
- Improved error handling in MergeExec so that the reason for threads failing is now propagated in an `Err`
- Removed unused `MapStruct` struct

Closes #7936 from andygrove/ARROW-9679

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