ARROW-11008: [Rust][DataFusion] Simplify count accumulator
ARROW-11008: [Rust][DataFusion] Simplify count accumulator

This PR makes the code to directly use `u64` instead of `ScalarValue`, simplifying some functions.

Closes #8991 from Dandandan/count_accumulator_simplify

Authored-by: Heres, Daniel <61924dac3230581b78b62e9e0cd0f8203281db27@gmail.com>
Signed-off-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>

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