Make @implementer and classImplements not add redundant interfaces

Fixes #204

Seen in plone.app.testing as a problem in Products.GenericSetup.

Added tests for that.

classImplementsOnly and @implementer_only can still be used to do that.

Only catch AttributeError instead of everything.

Fixes #200

Note that this does break a doctest in five.intid (it's expecting a TypeError but it now gets Acquisition's RuntimeError).

Fixes #197.

This is pretty, but it slows down all attribute access to interfaces.
By up to 25%. I'm not sure that's acceptable for things like
Interface.providedBy.

+-------------------------------------------+------------+-------------------------------+
| Benchmark                                 | 38-master3 | 38-faster3                    |
+===========================================+============+===============================+
| read __module__                           | 41.1 ns    | 44.3 ns: 1.08x slower (+8%)   |
+-------------------------------------------+------------+-------------------------------+
| read __name__                             | 41.3 ns    | 51.6 ns: 1.25x slower (+25%)  |
+-------------------------------------------+------------+-------------------------------+
| read __doc__                              | 41.8 ns    | 53.3 ns: 1.28x slower (+28%)  |
+-------------------------------------------+------------+-------------------------------+
| read providedBy                           | 56.7 ns    | 71.6 ns: 1.26x slower (+26%)  |
+-------------------------------------------+------------+-------------------------------+
| query adapter (no registrations)          | 3.85 ms    | 2.95 ms: 1.31x faster (-23%)  |
+-------------------------------------------+------------+-------------------------------+
| query adapter (all trivial registrations) | 4.59 ms    | 3.65 ms: 1.26x faster (-20%)  |
+-------------------------------------------+------------+-------------------------------+
| contains (empty dict)                     | 136 ns     | 55.4 ns: 2.45x faster (-59%)  |
+-------------------------------------------+------------+-------------------------------+
| contains (populated dict)                 | 137 ns     | 55.0 ns: 2.49x faster (-60%)  |
+-------------------------------------------+------------+-------------------------------+
| contains (populated list)                 | 40.2 us    | 2.95 us: 13.62x faster (-93%) |
+-------------------------------------------+------------+-------------------------------+

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.