When done from plugins, it doesn't correctly initialize the vtable
because of some weird things MSVC does, so the only safe way is to
use df::allocate<df::foo>(). For consistency, it is also enforced
for code in the main library. It reveals the issue in the digging
invaders plugin, first found by warmist.
This change is linked to a modification in df-structures codegen.
Alexander Gavrilov