This change includes a few steps towards AWS backend code-generation:
* Add a BoundDependencies property to ast.Stack to remember the *ast.Stack
objects bound during Stack binding.
* Make a few CloudFormation properties optional (cfOutput Export/Condition).
* Rename clouds.ArchMap, clouds.ArchNames, schedulers.ArchMap, and
schedulers.ArchNames to clouds.Values, clouds.Names, schedulers.Values,
and schedulers.Names, respectively. This reads much nicer to my eyes.
* Create a new anonymous ast.Target for deployments if no specific target
was specified; this is to support quick-and-easy "one off" deployments,
as will be common when doing local development.
* Sketch out more of the AWS Cloud implementation. We actually map the
Mu Services into CloudFormation Resources; well, kinda sorta, since we
don't actually have Service-specific logic in here yet, however all of
the structure and scaffolding is now here.
This change adds a Backend Phase to the compiler, implemented by each of the
cloud/scheduler implementations. It also reorganizes some of the modules to
ensure we can do everything we need without cycles, including introducing the
mu/pkg/compiler/backends package, under which the clouds/ and schedulers/
sub-packages now reside. The backends.New(Arch) factory function acts as the
entrypoint into the entire thing so callers can easily create new Backend instances.
