arguments are not memoized

Your documentation says that they are, but arguments are not memoized:

The script:

```
class Foo
  def bar(a)
    return a
  end
  memoize :bar
end

f = Foo.new
puts f.bar(1)
puts f.bar(2)
```

Outputs:

```
1
1
```

You need to keep a hash ivar that takes the array of args as a key, and calculated results as values.
