Implement a feature to check that a Solidity source did not change

[pirapira]

We have to get make compile_contracts fail when the source of SecretRegistry changes. For that we need a command that compares Soldity sources against an entry in contracts.json.

contracts.json contains "contract_checksums". This sounds useful for the purpose.

This issue can be closed when ContractSourceManager has a new method for this purpose.

[pirapira]

ContractSourceManager.verify_precompiled_checksums() does it, but only for all contracts at once.

In #1208, we need this feature only for SecretRegistry. We need a new method that verifies the checksum only for a specific contract.

[pirapira]

Fixing #1262 first makes a lot of sense here.