Better types

[andreiburdusa]

After #62

• replace abbrev UInt256 := Fin UInt256.size with

structure UInt256 where
  val : Fin UInt256.size

• make gas cost a natural number to avoid overflow

Extra: add 0x0a to π; add check for blob base fee

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Results:
Overall, not checking the balance and not having reconciled the expected errors yet, there are approximatively

• 279 tests blacklisted
• 21,788 tests running
• 19,360 tests succeeding

19,360 / (21,788 + 279) = 87.7%