blah.rb

require 'digest/md5'

def mixin(obj)
  m, k = Module.new {}, obj
  k.methods.each do |x|
    m.define_method(x, proc { |*args, &b| k.method(x).call(*args, &b) })
  end
  m
end

class Foo
  def initialize
    @bar = 1
  end

  def foobar
    [self, @bar]
  end
end

class Bar
  include mixin(Foo.new)
end

bar = Bar.new
p bar.foobar

class Baz
  include mixin(Digest::MD5)
end

baz = Baz.new
p baz.hexdigest('foo')