Add a really stupid warning about 'yield' used as an identifier.

Add a really stupid warning about 'yield' used as an identifier.

This is really stupid because it cannot be suppressed or altered using
the warning framework; that's because the warning framework is built
on Python interpreter internals, and the parser generator doesn't have
access to any of those (you cannot use anything of type PyObject * in
the parser).

But it's better than nothing, and implementing a proper check for this
appears to require modifying compile.c in a dozen places, for which I
don't have the stamina today.  I promise we'll do better in 2.2a2.

At least it tells you the filename and line number (unlike the first
hack I considered :-).