JavaKit support for static fields (wrapping Java types in Swift)

[DougGregor]

Right now, there is no way to express a static field in JavaKit. It should probably be represented as

extension JavaClass<TheSwiftType> {
  @JavaStaticField
  var theStaticField: Int32
}

We need to both introduce this macro (like with @JavaStaticMethod) and update the JavaToSwift tool to walk static fields of classes to emit these in the JavaClass extension, like it currently does for static methods.