feat(contracts): Use immutable variables for storage

[ratankaliani]

We only initialize storage variables once per implementation deployment. As such, we can use immutable variables to save on storage loads.

[ratankaliani]

For now, we've decided to table this feature, as the gas cost savings for a proposeL2Output call are at maximum ~4-5% and the immutable pattern is not recommended by OZ: https://docs.openzeppelin.com/upgrades-plugins/1.x/faq#why-cant-i-use-immutable-variables