Tighten down `Hash` JSON format

[Ericson2314]

As discussed on the call today:

1. We'll stick with format = "base16" and hash = "<hash>", not do base16 = "<hash>", in order to be forward compatible with supporting more versioning formats.

   The motivation we discussed for someday possibly doing this is making it easier to write very slap-dash lang2nix tools that create (not consume) derivations with dynamic derivations.

2. We will remove support for non-base16 (and make that the default, not base64) in Hash, so this is strictly forward contingency, not not yet something we support.