ARROW-7478: [Rust] [DataFusion] Group by expression ignored unless paired with aggregate expression
ARROW-7478: [Rust] [DataFusion] Group by expression ignored unless paired with aggregate expression

In addition to the change fixing the missing group by expressions, there were some code style changes that fixed some clippy warnings.

Closes #6115 from kyle-mccarthy/ARROW-7478 and squashes the following commits:

172bfa5c1 <Kyle McCarthy> Merge branch 'master' of github.com:apache/arrow into ARROW-7478
64e4e62ed <Kyle McCarthy> Fix: Missing Group by exprs

Authored-by: Kyle McCarthy <3c25b01657254677d3e1a8fd1f0742c5d489bd39@creativeanvil.com>
Signed-off-by: Neville Dipale <4e1ab218b24f06847d563ff3773c7517c139f863@gmail.com>

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