Support for mapping protocol and set objects

[iduartgomez]

We have support for the sequence, iterator and object protocol right now, but don't have support for the mapping protocol. Would be a nice addition to have in the future.

On the concrete object layer adding direct support for set objects would be a nice addition too (we have dictionaries, but not sets: https://docs.python.org/3.5/c-api/set.html).

I'm writing a wrapper which generates bindings automatically between rust and python leveraging type annotations and typing module for python 3.5 and I think sets are the only container type missing right now, as well as the mapping ABC (which would be supported adding mapping protocol).