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5.7.3 Protocol for automatic conversion to immutable

Sets can only contain immutable elements. For convenience, mutable `Set` objects are automatically copied to an `ImmutableSet` before being added as a set element.

The mechanism is to always add a hashable element, or if it is not hashable, the element is checked to see if it has an `__as_immutable__()` method which returns an immutable equivalent.

Since `Set` objects have a `__as_immutable__()` method returning an instance of `ImmutableSet`, it is possible to construct sets of sets.

A similar mechanism is needed by the `__contains__()` and `remove()` methods which need to hash an element to check for membership in a set. Those methods check an element for hashability and, if not, check for a `__as_temporarily_immutable__()` method which returns the element wrapped by a class that provides temporary methods for `__hash__()`, `__eq__()`, and `__ne__()`.

The alternate mechanism spares the need to build a separate copy of the original mutable object.

`Set` objects implement the `__as_temporarily_immutable__()` method which returns the `Set` object wrapped by a new class `_TemporarilyImmutableSet`.

The two mechanisms for adding hashability are normally invisible to the user; however, a conflict can arise in a multi-threaded environment where one thread is updating a set while another has temporarily wrapped it in `_TemporarilyImmutableSet`. In other words, sets of mutable sets are not thread-safe.