[Rust] make alloc module public

[asfimport]

Reporter: Ritchie / @ritchie46
Assignee: Ritchie / @ritchie46

PRs and other links:

• GitHub Pull Request #9572

_{Note: This issue was originally created as ARROW-11779. Please see the migration documentation for further details.}

[asfimport]

Andrew Lamb / @alamb:
Issue resolved by pull request 9572
#9572