Aggressively apply the const lint.

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

Add SliverList

A SliverList is a linear layout of box children in a viewport that all
have a common, fixed extent along the scroll axis. The layout is similar
to a SliverBlock but more efficient.

This structure makes it easier to customize the viewport used in a Scrollable.
Now Scrollable2 doesn't commit to using slivers.

When you put a box in a Viewport2, it might paint outside its allocated bounds.
This patch teaches Viewport2 to clip when that happens.