Fixes #21

The 'mostly' is because interfaces are used in cases that C3 forbids;
when there's a conflict, we fallback to the legacy algorithm. It turns
out there are few conflicts (13K out of 149K total orderings in Plone).

I hoped the fix for #8 might shake out automatically, but it didn't.

Optimize the extremely common case of a __bases__ of length one.

In the benchmark, 4/5 of the interfaces and related objects have a base of length one.

Fix the bad IROs in the bundled ABC interfaces, and implement a way to get warnings or errors.

In running plone/buildout.coredev and tracking the RO requests, the
stats for equal, not equal, and inconsistent-so-fallback, I got
{'ros': 148868, 'eq': 138461, 'ne': 10407, 'inconsistent': 12934}

Add the interface module to the Attribute str.

This was extremely helpful tracking down the Plone problem; IDate is defined in multiple modules.

Register implemented standard library types on import.

Derive the interface methods and documentation from the ABC automatically. I hope to use this for numbers too.

Part of #138

Explain that implements()/implementsOnly()/classProvides() not only
should not be "preferred", but are entirely not functional on Python 3.

(These notes should probably use some Sphinx directive to stand out
better, but I'm not well versed in Sphinx and I don't know which one I
should use.)

Add cross references and some formatting fixes.