Support LocalMapExtentExpr for Parakeet optimize

'visit_MapExpr` has been updated to keep the `extent` and `array`
dependencies even after optimization. Before, visit_MapExpr would lose
these two dependencies breaking user functions after optimization.

`add_dep` in local.py had to be updated to support three dependencies
(value, extent, array). Though ideally `array` will be dropped in a
later update.