[Python-3000] Abilities / Interfaces

[Bill Janssen]

> It may also be that only a good unified solution to "A" is needed in order to 
> allow library and user code to be written "easier" to address "B","C" and "D". 
> It may even be just doing "A" is a good enough 95% solution (works well enough 
> for 95% of actual use cases) and nothing more needs to be done. <It's too soon 
> to tell>

I guess this is sort of what I think.  And I hate to add yet another
framework/system to the Python system, thus my proposal to handle it
all using the existing type system, by re-factoring the various base
classes (file, sequence, number, list, mapping, string, etc.) into a
set of interface types, and re-defining the base types as various
combinations of these interface types (which may be abstract, or
concrete -- we haven't really talked enough about specifics to know
yet).

This would give us a decent base to use later (or now) for adding a
multi-method dispatching system, as well.  And optional type
annotations, etc.

Bill