Step circuit instance encoding solidity (with tests)

[willemolding]

Depends on #18 . Merge that first once it is ready.

• Adds a Solidity implementation of the instance hashing for the step circuit in a library
• Exposes this in a test contract so it can be deployed and tested
• Adds a new module to step.rs in the integration tests that ensures it exactly matches the Rust implementation
  • This uses automatically generated ABIs from the build artifacts and reads code from the out directory in contracts
• Adds CI task for building contracts as these are needed for the Rust tests now

There are only 2 files with changes that really need reviewing (contracts/src/SyncStepLib.sol and lightclient-circuits/tests/step.rs). The rest of the changes are mainly version bumps and changes to the contracts dir setup so it all works and builds again.

The provided tests should give immediate feedback if there are any changes between the two implementations in the future.

[nulltea]

Looks good! The question about the solidity version in generated contracts can be addressed separately or in the next PR