Move Deferred cancellation code out of trunk

[exarkun]

No one has actually tried to use this yet.  I still think it's probably a good
idea, but I don't think it should be part of the next release.

We should schwack it out into a branch and then bring it back when someone is
actually going to try to write some code that uses its functionality.

[radix]

Is this just a matter of doing some reversions of particular revisions? what are
they, if so?

[exarkun]

I think so.  svn log on defer.py should have some useful information.

[glyph]

For the record I will do this in the next few days, whenever I can find some time.

[radix]

Done at r14693.