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ARROW-7813: [Rust] Remove and fix unsafe code

This removes or corrects many instances of unsafe code in the rust crates. This is by no means a complete fix and some of the fixes do not entirely fix the issues with the particular `unsafe` (see comments), but in all instances it should put the code in a better place than before.

Based on #6256

Closes #6395 from Marwes/unsafe_rust_2

Authored-by: Markus Westerlind <markus.westerlind@distilnetworks.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>