Polish merge/beacon-chain.md with mostly non-substantive changes.

Non-substantive changes

• rename MAX_EXECUTION_TRANSACTIONS to MAX_TRANSACTIONS_PER_PAYLOAD
  • rename "execution transaction" to just "transaction" as per discussion with Danny
• rename compute_time_at_slot to compute_timestamp_at_slot
  • the function returns a Unix timestamp
  • "timestamp" matches execution_payload.timestamp
• be explicit about ExecutionEngine.execution_state for clarity
• rename ExecutionPayload.number to ExecutionPayload.block_number
  • more specific ("number" is pretty vague)
  • consistent with ExecutionPayload.block_hash
• rename new_block to on_payload
  • the on_ prefix is consistent with other event handlers (e.g. see on_tick, on_block, on_attestation here)
  • the _payload suffix is more to the point given the function accepts an execution_payload
  • avoids conflict with on_block which is already used in the fork choice
• rework the table of contents for consistency
• order is_execution_enabled after is_transition_completed and is_transition_block
  • is_execution_enabled refers to is_transition_completed and is_transition_block
• rename "transition" to "merge"
  • "transition" is a bit vague—we will have other transitions at future hard forks
  • there is no need for two words to refer to the same concept
• add a bunch of inline comments, e.g. in process_execution_payload
• make the process_execution_payload signature consistent with the other process_ functions in process_block which take as arguments state and block.body
• remove TRANSITION_TOTAL_DIFFICULTY
  • to be put in merge/fork-choice.md where it is used
• various misc cleanups

Substantive changes

• reorder ExecutionPayload fields
  • for consistency with yellow paper and Eth1
  • same for ExecutionPayloadHeader
  • added comments separating out the execution block header fields from the extra fields (cosmetic)