growing struct allocator calls the destructor on component removal
Merge branch 'rewrite' of github.com:TyOverby/Entreri into rewrite
Modified GrowingStructAllocator to not break pointers.

It is important that pointers don't get invalidated
in the middle of a system update.  We need to make
a promise that nothing will break in the interval
between a world.advance() and the next world.advance().

This commit adds this promise to GrowingStructAllocator.