Decouple warning detection from program instances

[HertzDevil]

This PR moves all the warning settings and detected warnings into a new Crystal::WarningCollection object. A new Crystal::Compiler or Crystal::Program comes with its own warning collection, but a collection may also be explicitly shared between them, which Compiler#new_program does. The sharing is needed because warnings from either component could trigger --error-on-warnings.

If we want to extend this to syntax warnings (see for example #12197 (comment) and #8373 (comment)), the treatment is also similar:

module Crystal
  class Parser
    property warnings : WarningCollection

    def initialize(..., @warnings = WarningCollection.new)
    end
  end

  class Compiler
    private def parse(program, source : Source)
      parser = Parser.new(source.code, program.string_pool, warnings: @warnings)
      # ...
    end
  end

  class Program
    def run(code, filename = nil, debug = Debug::Default)
      parser = Parser.new(code, warnings: @warnings)
      # ...
    end
  end
end

This gives the parser access to the compiler's warning settings, whereas a stand-alone parser is also able to maintain its own list of warnings without going through a program.

A Crystal::MacroInterpreter is never created alone, so it merely reuses its parent program's warning collection.