libxml-rs 0.1.0-alpha.6

Phase 5: XPath 1.0 engine, XPointer, XInclude. Native-Rust forensic reimplementation of libxml2+libxslt with C ABI drop-in replacement. 522 tests passing, full XPath 1.0 lexer/parser/AST/eval (25 functions), XPointer element scheme + shorthand pointers, XInclude process/process_flags, C ABI exports.