NSFileHandle: move {readability,writability}Handler

Since these are meant to be read/write properties as per Objective-C, this needs
to be a stored property.  However, it is not possible to have stored properties
in an extension.  Move the definition into the actual class definition.  Default
initialize to a closure that will invoke `NSUnimplemented`.

Author: compnerd

Foundation/FileHandle.swift

@@ -19,7 +19,14 @@ open class FileHandle : NSObject, NSSecureCoding {
     internal var _fd: Int32
     internal var _closeOnDealloc: Bool
     internal var _closed: Bool = false
-    
+
+    open var readabilityHandler: ((FileHandle) -> Void)? = {
+      (FileHandle) -> Void in NSUnimplemented()
+    }
+    open var writeabilityHandler: ((FileHandle) -> Void)? = {
+      (FileHandle) -> Void in NSUnimplemented()
+    }
+
     open var availableData: Data {
         return _readDataOfLength(Int.max, untilEOF: false)
     }
@@ -346,14 +353,6 @@ extension FileHandle {
     open func waitForDataInBackgroundAndNotify() {
         NSUnimplemented()
     }
-    
-    open var readabilityHandler: ((FileHandle) -> Void)? {
-        NSUnimplemented()
-    }
-
-    open var writeabilityHandler: ((FileHandle) -> Void)? {
-        NSUnimplemented()
-    }
 }
 
 extension FileHandle {