reimpl refs and scoping in Node

note: first step
refactor extract refs state from Node -> Refs

but the coupling is still nasty as hell
change Refs to simple data structure; dereferencing to be moved elsewhere

change Node to build internal Refs, no need for injection now
change: privatize Node#_refs
add tests for scoping and references
add back Schema.union
add missing each req in refs.js
change internal refs structure to array for multiple refs with same URI

update tests and add test for multiple refs
version bump; release edge to master branch