Improve clarinet's clarity linter

[smcclellan]

Linting rules can be configured to be “off”, “info”, “warning” or “error”.
Default value is indicated here between parenthesis.

• dead code
  • unused constant (warning)
  • unused function parameter (warning)
  • unused let binding (warning)
  • unused private function (warning)
• unsafe
  • avoid unwrap-panic (warning)
  • avoid unwrap-err-panic (warning)
• prefer native function aliases (introduced in clarity 2 and versions to come)
  • prefer element-at? (instead of element-at) (warning)
  • prefer index-of? (instead of index-of) (warning)
  • prefer bit-xor (instead of xor) (warning)
• convention
  • invalid naming case (info)

[obycode]

We could also add a check for a function that is defined as public but could be readonly. It should always be preferred to make the function read-only if it doesn't need to modify any state so that it is clear that it can be called via the API.