Refactor to remove compute_block_reward

- 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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