Shallow clean-up of the solidity code

[AntoineRondelet]

Related to: #321 and #192

This PR:

• Adjusts the Zeth solidity coding standards to rather align with the "language standards" proposed by the community and the solidity team. The driving factors for that are:
  • If there is a standard we should do our best to follow it (good practice) - it removes friction as to "what looks better" and just follows the norm.
  • Removes sources of inconsistencies between the code we write and the code we import (which catches the eye pretty badly when we call a function from an imported contract, like an ERC token for instance)
• Renames all solidity constants in UPPER_SNAKE_CASE
• Adds explicit scopes to all functions and storage variables (this should become systematic from now on as more verbosity with regard to the scopes can only be better)
• Switches the linter to solhint (solium then replaced by ethlint seems to be poorly maintained, last release was more than a year ago. solhint is actively maintained offers a good configuration set and is used by openZeppelin (https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/.solhint.json) and other serious projects). Several checks are voluntarily disabled for now because I didn't want to end up with a PR that shuffles the entire code base (renaming public functions, contract names etc => cascading these changes to the python code etc which will become a nightmare to review. Let's take it incrementally to avoid adding mistakes to the code base and breaking things)
• Uses NatSpec for documentation comments
• Orders functions (as per: https://docs.soliditylang.org/en/develop/style-guide.html#order-of-functions)
• Fixes the order of the modifiers

TODO:

• Fix the test_merkle_tree_contract.py test (we have a MerkleTreeMiMC7_test test contract but its import is broken since MerkleTreeMiMC7.sol does not exist anymore)
• Fix the test_contract_base_mixer.py test. It is now broken because the scope of some functions (e.g. test_assemble_nullifiers) has been changed in this PR, so these functions are not in the ABI anymore. Since we should not expose unnecessary function simply to be able to call them in the test scripts, let's add a MixerBase_test.sol "test proxy contract" to re-enable the tests despite the functions visibility change. - Fixed in Contract tests fix (depends on #333) #338

[AntoineRondelet]

@dtebbs there are some solidity variables that are seemingly unused. Could you double check this before we go further and consider removing them? (Are you using them somewhere I missed?)

[dtebbs]

@dtebbs there are some solidity variables that are seemingly unused. Could you double check this before we go further and consider removing them? (Are you using them somewhere I missed?)

@AntoineRondelet Yes, they do seem to unused. Good catch.

[dtebbs]

Looks good. Would be nice to have the CI catch violations. (I'll look into that)

[dtebbs]

I've tried to address my comments in #334

[AntoineRondelet]

Note: the test_contract_base_mixer.py test is broken. To properly fix it we would need to use a "test proxy contract", but this is for later (see #335). For now, let's just roll back and set the scopes of the tested functions back to public and update the tests to make sure that the tested functions behave correctly.

[dtebbs]

(Worth rebasing this onto origin/dvelop before merging)

[dtebbs]

#338 fixes the tests and provides public wrappers around the internal functions (so no need to adjust this code)