[Rust] Don't let safe code arbitrarily transmute readers and writers

[asfimport]

#6256

Reporter: Markus Westerlind / @Marwes
Assignee: Markus Westerlind / @Marwes

PRs and other links:

• GitHub Pull Request #6256

_{Note: This issue was originally created as ARROW-7775. Please see the migration documentation for further details.}

[asfimport]

Chao Sun / @sunchao:
Issue resolved by pull request 6256
#6256

[asfimport]

Wes McKinney / @wesm:
I added "[Rust]" to the issue title. This helps generate better changelogs