swift234

Property behaviors: Iterate prototype syntax to try a postfix form.

If behaviors are specified after the declaration, something like this:

  ```swift
  var x: Int __behavior foo // __behavior is a stand-in keyword
  ```

we're thinking this encourages a simpler design for smaller, more composable behaviors. If we think of behavior application as function-like, then parameters to the behavior could be passed with function-like syntax:

  ```swift
  __behavior lazy(@autoclosure initialValue: () -> Value) { ... }

  var x: Int __behavior lazy(1738)

  __behavior didSet(body: (oldValue: Value) -> Void) { ... }

  var x: Int __behavior didSet {
    trailingClosure()
  }
  ```

Since behaviors are implementation details, they arguably belong to the right of the declaration as well.