Add array and map types

[Geal]

as policies get more complex, we will see the need for more structured data, that could be queried from datalog. In particular, arrays and maps would make policies more flexible.

A few issues here:

• they require a list of operators supporting them
• the current syntax already uses [ ] for sets, which would be the common syntax to expect for arrays

[divarvel]

A few propositions:

Syntax

• use [] for lists
• use { <key>: <value> } for maps, where <key> and <value> can be an arbitrary datalog term
• use {} for sets

Possible issues: syntax ambiguity between parameters and sets (eg: {true}). we could remove set literals and provide set operators on lists (and remove Set from datalog), or keep Set, but only allow constructing it through a .toSet() method on lists.

New operators

• .get(<key>) on maps (where key can be any datalog term
• .get(<index>) on arrays (where index has to be a positive integer)

.get() on a map would return null for a key that is not in the map.
.get() on an array would throw an error on a key that is not an integer, and null if they key is negative or not in the array

Lifting restrictions

• lists and maps could be nested
• lists and maps could contain variables? not sure, we should investigate