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)`.

We're now using it at the widget layer for everything except scrolling and
flinging.

This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.

Fixes #743

Similar to widgets.dart, rendering.dart exports the entire rendering layer.
Also, update the examples to use rendering.dart and widgets.dart. Also clean up
some exports so that the examples have more sensible imports.

And use the service in mine_digger to notify the user when they flag a bomb.