Sharing hook

[TobiasWrigstad]

The default freezing behaviour of an object is to make all its fields immutable (including its class), and freeze all references in all fields. A freeze hook can be used to override this behaviour to replace all the contents of the fields before the fields and their contents are frozen, or to throw an exception.

[xFrednet]

Is this something we should definitly have for PLDI or an optional thing?