Add HexString/DottedQuad/Uuid support to IetfYangUtils

BGPCEP is instantiating a HexString through a rather funky construction involving hex dump, splitting and joining, where the input is a byte[]. For the inverse, it performs another pass, where it filters separators and uses javax.xml.bind.DatatypeConverter to arrive at a byte[].

Provide an appropriate utility method which will instantiate a HexString from byte[] with minimal overhead, bypassing validation. Also provide a utility method which will format a HexString into a byte[], again with minimal overhead.

The same goes for DottedQuad and Uuid (which should just bridge from java.util.UUID).