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This new lemma, contract_centric, restates proofs about single contracts
over some other sufficient lemmas involving strongly typed versions of
deployment info, state and messages. This makes proving those kind of
theorems much easier.

If the facts are wrong then this case will go wrong so it should be
proven first by the user.

This lemma makes proving simple properties about the deployed contract
state of a contract easier.

Also use these from Congress.v

Contracts cannot create blocks and balance of undeployed contract is 0

Remove it from the contract's perspective for now.

- No longer require that block reward can be computed from height.
- Remove Chain's dependence on BlockHeader. Instead inline appropriate
  fields in Chain structure.
- Change step_block to use a BlockHeader now instead of manually
  specifying all the fields. The new BlockHeader now additionally
  contains the creator and reward of that block, so step_block in effect
  contains the reward.
- These refactorings means that the circulation proof changes. Introduce
  created_blocks to get list of blocks created by user, and prove
  instead that the circulation equals the sum of rewards in blocks.
- Rename "baker" to a more general "creator" globally

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