Here's a patch that implements the new with statement
semantics: if __exit__ got passed an exception, it is
only swallowed when __exit__ returns *true*.  So it is
no longer necessary for __exit__ handlers to re-raise
the exception.

This is a bit rough still -- existing unit tests have
been adjusted, but there should be additional unit
tests for the new functionality.  Also the re-raise
code in various __exit__ handlers coded in the previous
patch could be removed (note that re-raising doesn't do
any harm -- it's just not necessary).