On-chain verification of increment and tests to cover all scenarios

[noonio]

What

• Implement the deposit validator and increment branch in the head validator
• Use the hydra-formal-specification as blueprint and "clear red areas" upon finished code changes
• All validator changes are tested using the mutation testing framework, covering all constraints from the specification and we saw each test "fail for the right reason", i.e. no specification change without a corresponding mutation and (at least) one mutation per constraint
• Test that the scenarios of Incremental commit #199 are covered by the L1 validation (not necessarily including L2 / off-chain code), i.e. using TxTraceSpec or similar tests