Allow skipping type checking

[soutaro]

Steep skips type checking right-hand side of an assignment to __skip__. This can be used to workaround writing Ruby code with an unsupported syntax.

Also introducing __any__ for more descriptive cast.

__skip__ = begin
  def object.foo
    # Singleton method definition is not yet supported by Steep.
    # It raises an error without the meaningless the assignment.
  end
end