Improve method of setting status_flags::dynallocated.

Improve method of setting status_flags::dynallocated.

There seems to be no way to obsolete the need to mark an object derived
from basic and handled by ex as being 'on the heap', at least none that
doesn't have significant performance impact. Having said that, this
patch aims at making this process simpler and more intuitive.

Where, before, one would return from a function returning an ex with
        return (new mul(a, b))->setflag(status_flags::dynallocated);
this patch lets us return with
        return dynallocate<mul>(a, b);
which should be much clearer. In any case, it involves less typing.

The two points where the status_flags::dynallocated are set are now
  * the dynallocate<B>(args...) template function and
  * the virtual duplicate() member functions.

This patch rolls out the new functionality throughout the library.