This reverts commit 7592213df29066cd357eaa4fffe4a19ed3bae189.

Turns out many of the functions on BoxConstraints weren't used or had callers
that could easily be updated to other functions. I've added dartdoc to all the
public functions as well as renamed some functions that had similar names but
did different things.

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

When laying out positioned children inside a stack, we should give them
unbounded constraints because if they draw outside of the stack, we'll just
clip them.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.

Also, make RenderStack support negative positions with clipping so that we can
expand objects that are partially offscreen.

Previously, we didn't set the width of the child correctly when top and bottom
(or left and right) were both specified.

Fixes #275