[Rust] Remove "array_ops" in favor of the "compute" sub-module

[asfimport]

Once 4591 (https://issues.apache.org/jira/browse/ARROW-4591) is complete only filter and limit will remain in the "array_ops" module and they can be moved under the "compute" sub-crate.

Reporter: Paddy Horan / @paddyhoran
Assignee: Paddy Horan / @paddyhoran

PRs and other links:

• GitHub Pull Request #5757

_{Note: This issue was originally created as ARROW-6881. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 5757
#5757