Remove the SizeObserver you need to use snap alignment in lists by
having the lists provide the size themselves in the callback.

Also, remove snapAlignmentOffset since we couldn't figure out how to
explain it and it's not really needed.

We use the ScrollDirection for more than just scrolling.

Fixes #151

ScrollableList2 is complete and subsumes all the use cases for the original
ScrollableList.

This patch also changed ScrollableList2 to use an Iterable instead of an
List for its children. This change lets clients map their underlying
data lazily. If the clients actually have a concrete list, we skip the
extra copy and grab the child list directly.

This package contains WidgetTester, which is very useful when writing tests for
widgets.

This seems useful for numbering the entries in the list, for example.

Fixes #328

Fixes #1084

Now that we've ported all the widget tests to fn3, we don't need a separate fn3
directory.