MODE-1956 Add helper function to (un)escape illegal JCR characters

_This incorporates and thus supersedes an [earlier pull-request](https://github.com/ModeShape/modeshape/pull/889)._

MODE-1956 Add helper function to (un)escape illegal JCR characters
MODE-1958 DDL Sequencer should use Jsr283Encoder rather than its own custom code

Added `encode` and `decode` methods to the `org.modeshape.jcr.api.Session` interface, and implemented these by simply calling through to the JSR-283 encoder/decoder. Also, removed the code from DDL sequencer framework dealing with converting JCR-illegal characters and replaced with calls to the new methods.