Backport improvements to set.py so that the interface will remain

Backport improvements to set.py so that the interface will remain
consistent across versions.

* Relaxed the argument restrictions for non-operator methods.  They now
  allow any iterable instead of requiring a set.  This makes the module
  a little easier to use and paves the way for an efficient C
  implementation which can take better advantage of iterable arguments
  while screening out immutables.

* Added a PendingDeprecationWarning for Set.update() because it now
  duplicates Set.union_update()

* Adapted the tests and docs to include the above changes.

* Added more test coverage including testing identities and checking
  to make sure non-restartable generators work as arguments.