Adapt Java char to a unicode object (jython#293)

This intends to fix jython#204 and jython#223 by making the conversion in the classic adapter (under Py.java2py) return unicode for char comparable to its behaviour with String.

In the process we rationalise the creation of unicode objects via the Py class, particularly for single-character strings, and also fix a bug latent in pickle.py whereby the wrong object is memoized in save_unicode().