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.
Is there going to be some way of reading the merged attributes? Like `merged_attributes[:fqdn]`?  Otherwise in my recipe I still have to remember which attributes are set in the `system` hash and which are set in the `attributes` hash.

EG, if I want to have some branching logic in my cookbook based on the state of a user, I have to remember whether I query that information from the `system` attributes or my `user` cookbook attributes.