Use HSet instead of CombinedEnv

[j-mueller]

• Use HSet (from hset) for environments
• Add HasEnv instances for HSet
• Delete the CombinedEnv types
• Move RuntimeEnv to offchain library - it was duplicated in both examples

Downside is that we can't use FunctionalDependencies for the operator environment anymore, which means that we need a couple of @era type annotations.

[amirmrad]

Much cleaner!
LGTM.