Extend 'in' operator to check for array subset.

Currently, the in operator does the following for a in b example:

• if b is a string, it checks if a is a substring of b
• if b is a list, it checks if a is an element of b

I propose that we extend the in operator to have the following additional behavior:

• if b is a list, and a is a list, then a in b should check if a is a subset of the elements in b

Examples:

• ["a", "b"] in ["a", "b", "c"] == true
• ["a", "d"] in ["a", "b", "c"] == false

The same goes for field references. Let's suppose the [tags] field is ["web", "frontend", "newyork", "staging"], then the following:

• ["web", "frontend"] in [tags] == true
• ["db", "newyork"] in [tags] == false

General idea is to have in act as a subset check if both operands are list values (array literals, fieldreferences that have array values, etc)