Unlike FractionalOffset, Alignment uses the center as the zero of the
coordinate system, which makes the RTL math work out much cleaner.

Also, make FractionalOffset into a subclass of Alignment so that clients
can continue to use FractionalOffset.

Notice that the default alignment for Stack needs to know the
directionality.

Fixes #11848

* Optimise AnimatedSize for the tight case.
* Remove `default` from a switch statement over enum (so that analyzer will complain if we add enum values).
* Adopt the Size since we use it after the child may have changed (which would throw normally).
* AnimatedCrossFade.layoutBuilder

* AnimatedCrossFade: shut off animations & semantics in faded out widgets

* address comments

The default is topCenter, which is sensible, but someone might want to use a
bottomCenter alignment, for example.

* more widget const constructors

* prefer const constructors

* address review comments

Fixed a bug where the size of the AnimatedCrossFade would always
start from the size of the first child, irrespective of the
initial crossFadeState argument.

Added a widget that cross fades two children while animating the
size of the parent based on the children's interpolated sizes.