Add a method in JsonGenerator for writing Object Id value/property?

[cowtowncoder]

Since some data formats (YAML, for example) have their own notions of Object Identity handling, it would make sense to push bit more semantical information through lower level abstractions. One way to do this would be to add new methods to use when serializing Object ids and references.

[cowtowncoder]

Implemented for 2.3.