Add the PureXML.Pattern namespace: a compiler for the streamable XPath
subset (names, *, prefix:*, /, //, leading absolute /, trailing @attr) into
a Matcher, with predicates and . / .. rejected. The backward-matching
algorithm tests an element's ancestor-or-self path. Pattern.matches(_:in:)
streams a document through the pull parser and returns matching node paths
in document order from the open-element stack alone, no tree built.
Closes#44Closes#45Closes#22
