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

We now have Mimic tracking Mimicable through tree structure changes and while
moving around the screen.

Fixes #751 and #756

Block -> BlockBody
ScrollableBlock -> Block
FixedHeightScrollable -> ScrollableWidgetList
VariableHeightScrollable -> ScrollableMixedWidgetList
BlockViewport -> MixedViewport

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.

This widget lets you mimic one of its children in an overlay. The overlay
starts out as the same size of the child and then grows to fill the overlay. In
the future, the mimic will start at the same visual position as the child.

Wrap widgets you want to reparent in a Mimicable widget and assign the
Mimicable widget a global key. Then, given the same global key to a Mimic
widget to make it appear elsewhere in the view hierarchy.