add implicit reference string schemas

[gregsdennis]

This is just a concept PR to get an idea of what #1078 might look like.

I'm not too particular on the wording of the new core spec section, but it's interesting to see the effect on the meta-schemas. It definitely eliminates the cruft of having to wrap every reference in { "$ref": ... }.

There is an argument to be made about readability, but I'm sure it's just a matter of getting used to seeing it (like boolean schemas was).