class T::Types::TypeVariable

provides some syntax for the static type checker
Since we do type erasure at runtime, this just validates the variance and

def initialize(variance) 

def initialize(variance)
  if !VALID_VARIANCES.include?(variance)
    raise TypeError.new("invalid variance #{variance}")
  end
  @variance = variance
end

def name 

def name
  Untyped.new.name
end

def subtype_of_single?(type) 

def subtype_of_single?(type)
  true
end

def valid?(obj) 

def valid?(obj)
  true
end