Fix on-chain security and test it throughly

[uhbif19]

TODO on-chain checks:

• Check stakes
• Forbid minting by default
• Check coin-change is correct
• Check TxOut markings

TODO test for common constraint bugs (apart from testing checks):

• Tx ordering and coin selection does not matter

Blocked by #24 #48

[github-actions]

Status: Ready to merge ✔️

Issues blocking this PR:

• Implement basic static DSL for constraints #24 ✔️

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