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We currently allocate instance values when creating the instance itself
(except when constructing the instance for a `cast()`), but there is no
particular reason to do so: the instance itself and the internals (for
a non-simple layout) are allocated via Python, with no reason to
expect better locality from the invoked `operator new`.  Moreover, it
makes implementation of factory function constructors trickier and
slightly less efficient: they don't use the pre-eallocate the memory,
which means there is a pointless allocation and free.

This commit makes the allocation lazy: instead of preallocating when
creating the instance, the allocation happens when the instance is
first loaded (if null at that time).

In addition to making it more efficient to deal with cases that don't
need preallocation, this also allows for a very slight performance
increase by not needing to look up the instances types during
allocation.  (There is a lookup during the eventual load, of course, but
that is happening already).