bugs #126161 and 123634).

The solution doesn't use the unicode-escape encoding; that has other
problems (it seems not 100% reversible).  Rather, it transforms the
input Unicode object slightly before encoding it using
raw-unicode-escape, so that the decoding will reconstruct the original
string: backslash and newline characters are translated into their
\uXXXX counterparts.

This is backwards incompatible for strings containing backslashes, but
for some of those strings, the pickling was already broken.

Note that SF bug #123634 complains specifically that cPickle fails to
unpickle the pickle for u'' (the empty Unicode string) correctly.
This was an off-by-one error in load_unicode().

XXX Ugliness: in order to do the modified raw-unicode-escape, I've
cut-and-pasted a copy of PyUnicode_EncodeRawUnicodeEscape() into this
file that also encodes '\\' and '\n'.  It might be nice to migrate
this into the Unicode implementation and give this encoding a new name
('half-raw-unicode-escape'? 'pickle-unicode-escape'?); that would help
pickle.py too.  But right now I can't be bothered with the necessary
infrastructural changes.