Defines one or more abstract methods with given names in a class or module. When called, the abstract method will raise an `AbstractMethodCalled` exception with a helpful message.

@example

  class AbstractClass
    abstract_method :foo
  end

  class ConcreteClass < AbstractClass
    def foo
      42
    end
  end

  AbstractClass.new.foo # raises AbstractMethodCalled
  ConcreteClass.new.foo # => 42

@param [Array<Symbol>] names the names of defined abstract methods