Grammars and unmarshallers

To be written...

describe the ideas presented in the heading comments in

https://github.com/jsalvata/scala/blob/68d02868684082cd2f762960b093d6a75893a5aa/src/library/scala/xml/ScalaXMLUnmarshaller.scala

(for regular "Greibach NF" grammars)

https://github.com/jsalvata/scala/blob/typed-xml/src/library/scala/xml/ScalaXMLUnmarshaller.scala

(for more generic grammars, including Chomsky NF)