[Cython] Speed up cython for enum declarations

[Stefan Behnel]

Robert Kern wrote:
> It looks like a fair chunk of time is getting spent by the getattr() on line 26 
> of Visitor.py:BasicVisitor.visit() because of missing entries in the 
> dispatch_table.

BTW, I just noticed that I forgot to push my fix for that code section to
the release branch. There's a bug in there that renders the dispatch_table
mostly useless.

If you try with cython-devel, it should compile a *lot* faster.

Stefan