conversion of Sets

The Julia `Set` type should be translated to a Python `set`, possibly with a `PySet` (like `PyDict`) to allow copy-free sharing.

It would help if I knew of an actual use-case for this. So far, no one has complained about PyCall not converting sets.

(Moved to its own issue from #9.)

Start by examining how PyCall currently converts Julia collections to Python objects, using the existing PyDict behavior mentioned in the issue as a comparison point. Determine whether Python set conversion alone is sufficient or whether a PySet for copy-free sharing is needed; done means the supported Set behavior is defined and covered by the relevant existing tests.

Written by the indexing model from the issue text.