Support arrays

[winterstein]

Arrays as input: Does this just work already? With the indices treated as map keys?

Arrays as output -- either it automatically chooses to keep numerical keys in an array, or a syntax to specify that, e.g. "v[]" would create an array of vs, and "v[i]" would create an array of vs using the index i.
Or would "[]i -> v" be a better syntax, being closer to "i -> v"?

[winterstein]

So array inputs just work. But they get converted to maps on the output.