- Made some fixes to the system of producing custom instrumented

- Made some fixes to the system of producing custom instrumented
collections, mainly that the usage of the @collection decorators
will now honor the __mro__ of the given class, applying the
logic of the sub-most classes' version of a particular collection
method.   Previously, it wasn't predictable when subclassing
an existing instrumented class such as :class:`.MappedCollection`
whether or not custom methods would resolve correctly.
[ticket:2654]

- The undocumented (and hopefully unused) system of producing
custom collections using an ``__instrumentation__`` datastructure
associated with the collection has been removed, as this was a complex
and untested feature which was also essentially redundant versus the
decorator approach.   Other internal simplifcations to the
orm.collections module have been made as well.