Allow assignment to list / map fields from any implementation of sequence / mapping protocols

[kamalmarhubi]

Check against the Sequence and Mapping ABCs from collections.

From @jparyani on a mailing list discussion:

add them to the bottom of the if/else type checking chain since isinstance takes close to 100x as long as the current type checking. Take a look at https://github.com/jparyani/pycapnp/blob/a5bf532d53db37414d5e6b25f139ef237a854cc1/capnp/lib/capnp.pyx#L686 to see the logic for this. While you're at it, we should add the same for dict->collections.Mapping.

[kamalmarhubi]

I don't seem to be able to assign the issue to myself, but I'm doing this.