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.
Fixes a crash when property behavior uses are run through the specializer, since they set up associated type conformances that are not recoverable by name lookup.
If a behavior protocol requires an `initialValue` static property, satisfy the requirement using the initial value expression from the property declaration. This lets us implement `lazy` as a property behavior.
Since the feature is incomplete and yet to be accepted or implemented as proposed, hide it behind an -enable-experimental-property-behaviors frontend flag.
