This change slightly refactors the way resources are created and
implemented. We now have two implementations of the Resource interface:
* `resource` (in resource_value.go), which is a snapshot of a resource's
state. All values are resolved and there is no live reference to any
heap state or objects. This will be used when serializing and/or
deserializing snapshots of deployments.
* `objectResource` (in resource_object.go), which is an implementation
of the Resource interface that wraps an underlying, live runtime object.
This currently introduces no functional difference, as fetching Inputs()
amounts to taking a snapshot of the full state. But this at least
gives us a leg to stand on in making sure that output properties are
read at the right times during evaluation.
This is a fundamental part of pulumi/lumi#90.
2ab2b09474