Attribute API 2

**NOTE:** This RFC is incomplete, the specification of the new feature is TBD.

This RFC describes a new attributes API to be added alongside the current one. The goals of this new API are:

* Separate desired and observed state.
* Structure the syntax in a way that "trainwreck" errors
  (`NoMethodError` for `[]` on `nil`) can be avoided.
* Structure the syntax in a way that attribute tracing can be
  implemented simply.
* Separate node-specific persistent storage from
  cookbook/role/environment attributes.
* Structure the syntax in a way that can be implemented efficiently.

This RFC is adapted from https://github.com/opscode/chef-rfc/pull/9 which is retracted.
Also, you could use element assignment along with namespace blocks (using a block variable) like:

```ruby
defaults_for("nginx") do |n|
  n["foo"] = "bar"
end
```

I personally disfavor the element reference stuff because it looks too much like the old syntax to me, so I think it makes transitioning to the new thing confusing.