PEP 308 implementation, including minor refdocs and some testcases. It

PEP 308 implementation, including minor refdocs and some testcases. It
breaks the parser module, because it adds the if/else construct as well as
two new grammar rules for backward compatibility. If no one else fixes
parsermodule, I guess I'll go ahead and fix it later this week.

The TeX code was checked with texcheck.py, but not rendered. There is
actually a slight incompatibility:

>>> (x for x in lambda:0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: iteration over non-sequence

changes into

>>> (x for x in lambda: 0)
  File "<stdin>", line 1
    (x for x in lambda: 0)
                     ^
SyntaxError: invalid syntax

Since there's no way the former version can be useful, it's probably a
bugfix ;)