If there are stack dependencies, 'diff' would fail because it does not
know how to diff multiple stacks. Make diff support the same stack
selection mechanisms as 'cdk deploy'.

Move 'stack rename' facilities into the class that deals with the
CDK app, which is the source of thruth for stacks. This way, all
downstream code doesn't have to deal with the renames every time.

Start factoring out toolkit code into logical layers. Introducing
the class `CdkToolkit`, which represents the toolkit logic and
forms the bridge between `AppStacks` which deals with the CDK
model source (probably needs to be renamed to something better)
and `CfnProvisioner`, which deals with the deployed stacks.

N.B.: The indirection to a provisioner class with an interface is
because the interface is going to be complex (therefore, interface
over a set of functions that take callbacks) and we want to depend
just on the interface so it's easy to stub out for testing.

…k-diff