IMR objects may own memory. object_allocator takes care of allocating
memory for all owned objects during the serialisation of their owner.
In practice a writer of the parent object would accept a helper object
created by object_allocator. That helper object would be either
responsible for computing the size of buffers that have to be allocated
or perform the actual serialisation in the same two phase manner as it
is done for the parent IMR object.
In some cases the actual value of an IMR object is not know at the
serialisation time. If the type is fixed-size we can use a placeholder
to defer writing it to a more conveninent moment.
This patch introduces destructors and movers for IMR objects which
enables them to own memory. Custom destructors and methods can be
defined by specialising appropriate classes.
