Allow unsafe virtual-docs about opa

[tsandall]

I wonder if this can be handled using the "with" keyword.

For instance, in the example above we would rewrite prohibited as prohibited :- input.path[0] = "bar", input.path[1] = "baz". To apply to arbitrary values, the "with" keyword could be used: not prohibited with input.path as ["foo", "bar"]. Note, the value could come from input, data, literals, etc.

One issue with this suggestion is that (currently) we return an error if the caller supplies an input document to query that already defines one using the "with" keyword. This would need to be relaxed.

[tsandall]

Fixed by #345