Add Lint/UselessDef and Lint/UselessVisibilityModifier

[nobodywasishere]

This may be something that should be a compiler warning instead, so feel free to close if it should be. It's possible to define operator methods top-level that are impossible to call:

def +(other)
  p! other
end

+1 # => 1

As well, it's possible to define a method with protected top-level, when the visibility modifier has no effect

protected def hello(a)
  p! a
end

hello(1)
# a # => 1

See: crystal-lang/crystal#12360

[straight-shoota]

Since these methods cannot be called and the visibility restriction makes no sense, I suppose this could find its place in the compiler. Feel free to start adopting it in ameba though.

[nobodywasishere]

My personal preference for these (and other things that would give compiler warnings) is to have them in both, due to ameba's integration with dev environments being better + a better system for reporting issues.

[Sija]

I also think it's a good idea to have these rules.

[nobodywasishere]

I'll take a look at implementing these once we get through the current set of PRs.