[R] convert Array of LIST type to R lists

[asfimport]

Reporter: Romain Francois / @romainfrancois
Assignee: Romain Francois / @romainfrancois

PRs and other links:

• GitHub Pull Request #4575

_{Note: This issue was originally created as ARROW-5586. Please see the migration documentation for further details.}

[asfimport]

Romain Francois / @romainfrancois:
Issue resolved by pull request 4575
#4575