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?