[abstract interpreter] re-enable handling of arrays

Summary:
Mostly mechanical. This adds back in our original modeling of
arrays. String literals are mapped precisely, while integer indices
are less precise (based on my superficial understanding).

Test Plan: Will add corresponding integration tests.

Reviewers: pad

Reviewed By: pad

CC: collingreene

Differential Revision: https://phabricator.fb.com/D1262073