Re-level dependency tiers

[joshsh]

Currently, we have a system of 6 dependency tiers within the Hydra code base, as described here: tier-0 for kernel type definitions and selected constants, tiers 1 through 3 for other kernel code, tier-4 for tests and language modules, and tier-5 for third-party code. This layer cake is a little too thick IMO, and could probably be compressed. In particular, tier-3 is almost empty and could probably be combined with tier-2. Also, I don't think there is much value in distinguishing the built-in language modules from third-party code in terms of dependencies. Finally, it is a little odd that Hydra Core isn't properly in any tier, including tier-0. Here is the plan going forward:

• tier-0: Hydra Core + independent primitive functions
• tier-1: kernel type definitions + selected constants + selected logic and primitive functions which depend on tier-0
• tier-2: kernel logic and primitive functions which depend on tier-1
• tier-3: kernel logic and primitive functions which depend on tier-2
• tier-4: everything outside of the Hydra kernel

Tier-4 isn't so much a single tier as an "everything else" bucket, regardless of dependency structure.