This improves a few things about assets:
* Compute and store hashes as input properties, so that changes on
disk are recognized and trigger updates (pulumi/pulumi#153).
* Issue explicit and prompt diagnostics when an asset is missing or
of an unexpected kind, rather than failing late (pulumi/pulumi#156).
* Permit raw directories to be passed as archives, in addition to
archive formats like tar, zip, etc. (pulumi/pulumi#240).
* Permit not only assets as elements of an archive's member list, but
also other archives themselves (pulumi/pulumi#280).
This change fixes the serialization of resource properties during
deployment checkpoints. We erroneously serialized empty arrays and
empty maps as though they were nil; instead, we want to keep them
serialized as non-nil empty collections, since the presence of a
value might be semantically meaningful. (We still skip nils.)
Also added some test cases.
