The "strange" code is a copy of PyUnicode_DecodeUnicodeEscape. I find it
easier to read. And the duplicate lines are likely to be optimized by
the compiler.

Here is a new version of the patch which:
- correctly forbid illegal code points
- compute the byte positions; this is important for error handlers

in python2.5, the end position was completely bogus:
>>> try: '\U11111111'.decode("raw-unicode-escape")
... except Exception, e: print repr(e)
UnicodeDecodeError('rawunicodeescape', '\\U11111111', 0, 504955452,
'\\Uxxxxxxxx out of range')