This change simply adds the necessary AWS CloudFormation struct types to
permit marshaling and unmarshaling to/from JSON and YAML. This will be used
by the AWS cloud provider's backend code-generation. It adds a bit of strong
typing so that we can catch more errors at compile-time (both for our own sanity
but also to provide developers better diagnostics when compiling their stacks).
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.
