Support simple list cases in FieldPolicyCacheResolver

[BoD]

Handle simple cases like:

type Query {
  books(ids: [String!]!)
}
extend type Query @fieldPolicy(forField: "books", keyArgs: "ids")

where

• the field is a flat list
• keyArgs is a single arg which is a flat list

Other cases fall back to what we had previously.

See #142.