Allow passing block to Data definition

[stephannv]

From Ruby docs:

The native Data ::define method accepts an optional block and evaluates it in the context of the newly defined class. That allows to define additional methods:

Measure = Data.define(:amount, :unit) do
  def <=>(other)
    return unless other.is_a?(self.class) && other.unit == unit
    amount <=> other.amount
  end

  include Comparable
end

This will be useful for Literal when it is necessary to define custom properties/methods inside Literal::Data.define, eg:

Success = Literal::Data.define(user: User) do
  def success? = true
  def failure? = false
end

Failure = Literal::Data.define(reason: String, user: User) do
  def success? = false
  def failure? = true
end