* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

Importing the public libraries caused a name conflict with dart:sky because we
assume people will import dart:sky into a namespace, so I've also changed
skysprites to import dart:sky into a namespace.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.

What's important about this code is that it's presenting services outside the
VM, not the particular technology used to present the services.

We now expose idiomatic setters for these properties. Eventually we'll remove
the setter functions.

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.

This still leaves Flex and FlexDirection available. At some point once
people have transitioned to Row/Column we should rename Flex to _Flex
and stop reexporting FlexDirection from basic.dart.