ARROW-10889: [Rust] [Proposal] Add guidelines about usage of `unsafe`#8901

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jorgecarleitao wants to merge 6 commits intoapache:masterfrom jorgecarleitao:arrow_unsafe

+79

Commits on Dec 14, 2020

Commits on Dec 16, 2020

Typos and incorporated changes.
jorgecarleitao
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