Change compact_pointer_parent to use 2 bytes
Change compact_pointer_parent to use 2 bytes

Parent pointers need to be able to reach everywhere within a page to
minimize shared parent pointer reuse unless it's absolutely necessary.
This reduces parent hash utilization on all test cases to <1%.

Rename compact_parent to compact_shared_parent.

zeux/pugixml,zeux/pugixml,zeux/pugixml,zeux/pugixml