Change `TokenBuffer` to use new `writeEmbeddedObject()` if possible

[cowtowncoder]

(for background, see: michel-kraemer/bson4jackson#67)

Newly (2.8) added JsonGenerator.writeEmbeddedObject() should be useful alternative to writeObject(), so if possible, TokenBuffer should use it. But as-is there may some issue wrt non-support by default generator implementation.