Rationalise the Key API.

Add a way of having keys based on numeric types or DateTimes by having a ValueKey<T> class.
Remove the redundant ways of declaring things, except for leaving one shorthand -- you can say `new Key(s)` instead of `new ValueKey<String>(s)`.

engine/src/flutter/examples/mine_digger/lib/main.dart

@@ -142,7 +142,7 @@ class MineDiggerApp extends App {
         new Row(
           row,
           justifyContent: FlexJustifyContent.center,
-          key: new Key.stringify(iy)
+          key: new ValueKey<int>(iy)
         )
       );
     }