More accurate return type in MetadataStandard.getInterface/getImplementation(...) methods,
and added a trivial Characters.isHexadecimal(int) method to be needed later for a JSON parser.


git-svn-id: https://svn.apache.org/repos/asf/sis/branches/JDK8@1617481 13f79535-47bb-0310-9956-ffa450edef68