Change spelling of "direct" subscript overload on Entity.

⚠️ Breaking change ⚠️

Take the following definitions:

enum TestDescription: EntityDescription {
  static var jsonType: String { return "test" }

  typealias Relationships = NoRelationships

  struct Attributes: JSONAPI.Attributes {
    let normal: Attribute<String>
    var direct: String { return normal.value }
  }
}
typealias Test = Entity<TestDescription, NoMetadata, NoLinks, String>

let test: Test = ...

You used to be able to access both attributes using the default subscript:

let one: String = test[\.normal]
let two = test[\.direct]

However, in the case of test[\.normal], the code was ambiguous unless you explicitly asked for a String (the compiler would not know if you wanted the String or the whole Attribute<String>).

That is what has changed. There is no more ambiguity, but now the direct access subscript has a new spelling:

let one = test[\.normal]
let two = test[direct: \.direct]

It may seem like the verbosity of the code has just moved from one place to another, but the much less common feature (direct access) has taken on the burden.