2. Replaced calls to Set([]) with Set() -- Timbot's suggestion
3. Fixed subtle bug in sets of sets:

The following code did not work (will add to test suite):
    d = Set('d')
    s = Set([d])  # Stores inner set as an ImmutableSet
    s.remove(d)   # For comparison, wraps d in _TemporarilyImmutableSet

The comparison proceeds by computing the hash of the
_TemporarilyImmutableSet and finding it in the dictionary.
It then verifies equality by calling ImmutableSet.__eq__()
and crashes from the binary sanity check.

The problem is that the code assumed equality would be checked
with _TemporarilyImmutableSet.__eq__().

The solution is to let _TemporarilyImmutableSet derive from BaseSet
so it will pass the sanity check and then to provide it with the
._data element from the wrapped set so that ImmutableSet.__eq__()
will find ._data where it expects.

Since ._data is now provided and because BaseSet is the base class,
_TemporarilyImmutableSet no longer needs .__eq__() or .__ne__().

Note that inheriting all of BaseSet's methods is harmless because
none of those methods (except ones starting with an underscore)
can mutate the .data element.  Also _TemporarilyImmutableSet is only
used internally as is not otherwise visible.