Validate XSD identity constraints over the instance document in a pass that
walks the tree alongside type validation:
- xs:unique requires the selected nodes' field tuples to be distinct, excluding
any node that is missing a field.
- xs:key additionally requires every field to be present.
- xs:keyref requires each tuple to equal a tuple of the named key or unique
visible in an enclosing scope, resolved through a stack of scope frames so a
keyref and the key it refers to may sit on the same element.
The selector and field XPaths run on the existing XPath engine, relative to the
declaring element and each selected node. Constraints are gathered for every
element declaration at any depth, keyed by element name.
Closes#67
