Switch from Solhint to Slither

[CodeSandwich]

solhint isn't a very good linter anymore:

• It's not very helpful. It very rarely finds anything and I can't recall a single time when its warning actually improved the code quality, these were always annoying nitpicks which forced flagging code to be ignored.
• It's basically abandoned. The repository hasn't been pushed to in over a year and minimalistic Solidity-bumping updates are released by a single person without access to the upstream.

Slither seems to be an alternative worth trying out. It's actively developed and seems to be widely praised among developers.

[CodeSandwich]

Blocked by https://github.com/crytic/slither/issues/1352 (being fixed in https://github.com/crytic/slither/pull/1378), unless it's implemented, Slither can't analyze this codebase.