Don't export keys for non-associative arrays

Summary: phutil_var_export can produce cleaner output if keys are omitted for non-associative arrays. Depends on D9935.

Test Plan: arc unit

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9936