An array should be an iterable expression

Right now, only `range` and `enumerate` produce an iterable expression (according to the DSLX reference). An array should also produce an iterable expression. We would have this blueprint:
```
for (array_value, accumulator): (type-of-element_of_some_array, type-of-accumulator) in some_array {
body-expression
} (initial-accumulator-value)
```

I assume this could build on the existing support for `enumerate`, and discard the part that produces the indices of the array.

----

Extension: bit slices also strike me as something iterable (but for now, I'm more in need of the array support).