Simplify encoding handling of ntriples unescape.

All ntriples data has UTF-8 encoding, according to the standard.

When referencing unicode code points with \u or \U, there seems to be no
need to convert the string to binary, all code points can be represented in UTF-8.
Add deprecation warnings for use of Surrogate Pairs and other areas previously commented as being deprecated:

* URI subject to list (other than rdf:nil)
* Writer#write_graph
* Writer#write_statements

This relates to issue #163.