* YamlParserEvent -> YamlStreamEvent
* YamlStreamEvent is no longer a `ref` object. Test suite now
runs in a small fraction of a second (compared to several
seconds before)
* Introduced YamlStream
* Parse alias nodes properly
* Fixed a bug that prevented you from having objects as map keys
* Use AnchorId for reporting anchors instead of strings
* Do not use nil strings for anchor and tag
* introduced YamlSequentialParser object that stores
all known tag URIs
* Added procs for registering and querying tag URIs
* Return non-specific tags instead of nil if no tags
exist in source
* Lexer: column now always points to start of token
* Lexer: added ability to compile with -d:yamlDebug for debug output
* Parser: Correctly handle nested sequences and maps
* Tests: Added parser tests for nested objects