General Contract Cleanup + Write out style guide

[LHerskind]

Write out the styleguide that Jan and I used for Aztec connect + update it and see if we can use https://github.com/0xKitsune/solstat to enforce as much of it as possible (or add extra rules).
If solstat requires too much work to get to a good state for us, consider using https://github.com/LHerskind/solhint which has many of the rules, but relies on gettign node into the contracts part again 😓.

Update the contracts to follow the style guide.

[LHerskind]

Adding linter and performing some general cleanup in #667 and #674.