Support attr_accessors for singleton class

[tk0miya]

To support attr_accessor, reader and writer for singleton class, this converts them to the mere method definitions.

class Foo
  class << self
    attr_accessor :bar
  end
end

The above code will converted into the followings:

class Foo
  %a{pure}
  def self.bar: () -> untyped
  def self.bar=: (untyped) -> untyped
end

[soutaro]

@tk0miya Thank you for opening this PR. I guess we can just use attr_reader self.bar: untyped syntax?

[tk0miya]

OMG! I don't know such a strong syntax. Now I fixed!