Partial rules defining objects must have json string keys in head

[patrick-east]

Expected Behavior

In rego a valid object can have any type of key. Ex:

x := {
  ["a", "b", "c"]: "foo"
}

Is perfectly valid.

Building objects with partial rules should therefore allow for constructing the same type of objects like:

p[["a", "b", "c"]] = "foo" { true }

Actual Behavior

You get a parse error:

> p[["a", "b", "c"]] = "foo" { true }
|
1 error occurred: 1:1: rego_parse_error: object key must be string, var, or ref, not array
	p[["a", "b", "c"]] = "foo" { true }

But you can define it like...

> p[x] = "foo" {
|   x := ["a", "b", "c"]
| }
|
Rule 'p' defined in package repl. Type 'show' to see rules.

Additional Info

These types of objects are somewhat dangerous since you cannot serialize them into JSON for results. However for intermediate data structures in a policy they are supported, and it seems weird that we restrict the syntax.