[Feature] `#unwrap_optional` helper

[amomchilov]

It's a really common operation to unwrap the optionality out of a type, e.g. turning String? into String.

I'd propose (in psuedocode):

interface RBS::Types::_TypeBase
  # @overridable
  #: -> _TypeBase
  def unwrap_optional
    self # not optional, nothing to unwrap.
  end
end

class RBS::Types::Optional
  # @override
  #: -> _TypeBase
  def unwrap_optional
    type
  end
end