Anchor and TagDirectives in separate threads anymore. D:YAML
still is not thread safe itself, though (it's the user's
responsibility to handle synchronization when working with a
single node from two threads, for example).
Removed endMark from Node to keep it in 32 bytes on 64bit.
This will result in slightly worse debugging messages, but we
still have the start position of a node.
Tag is needed for better compliance with the spec and emitting
support for multiple tags with the same D data type.