update RelaxNG schema for -07:

* break "char-single" into "char-simple" and "range-simple" per
* suggestion from Asmus Freytag
* new attribute from-tag
* rename attribute byref => by-ref
* class-ref (as used in <class by-ref="">) is now xsd:IDREF
* clarified tags' identifier-like syntax per Asmus' suggestion
* define pattern for code-point-set-shorthand
* define pattern for range-shorthand
* fix meta-section thanks to Paul Hoffman for spotting issue