[Yaml-core] Inheritance Key

[Brian I. <in...@tt...>]

Now that Oren has announced the last call spec, I think it's time to suggest
some new proposals. THAT DON'T AFFECT THE SPEC!! :)

The reason for moving all types out of the spec is so that we can discuss,
add, delete, and modify types without changing the spec. Loaders are
encouraged to support types in the repository but are not required to. In the
absence of a schema or an explicit type, a loader can load any node as it best
sees fit. It should, however, attempt to adhere to the YAML models as well as
possible.

ANYHOW, Oren and I discussed the YAML schema stuff a bit, and found a need for
a special key called the inheritance key. This key has a value that is a
boilerplate mapping for the node. It is very useful for annotating things that
are similar but different. (like schema nodes :)

    ---
    blue: &BLUE
      r: 0
      g: 0
      b: 255
    light-blue:
      <: *BLUE
      r: 128
      g: 128
    ---
    father: &INGERSON
      name: 
        first: Jimmy
	last: Ingerson
      address:
        street: 123 Main
	city: Springfield
	state: NY
    brother:
      <: *INGERSON
      name:
        first: Sue
    ...
      
Oren had suggested '<=' but I think '<' is cleaner.

Thoughts?

Brian