Add Schema.Document.validate(streaming:) and ComplexValidator.validateShallow:
resolve each element's type from its parent's content model (and xsi:type /
typeReference) as it opens, then check its attributes and content-model
structure shallowly as it closes, against a shallow element synthesized from
the streamed child names. A child's deeper validity is checked when it closes,
so only the open-element stack is held. A differential test asserts streaming
agrees with the tree validator across valid and invalid documents (missing
attributes, bad simple values, missing/extra/misordered children). Wildcard
processContents and substitution-group resolution in the streaming form stay
on the tree validator. The streaming methods moved to ComplexValidatorStreaming
to keep ComplexValidator under the file/type-body caps.
Closes#107
