-registering same system multiple times only replace pointer for callback

-added export attributes (required by windows to make DLL library, not work at now due to DMD bugs)
-interface D files to import

import/ecs/entity_allocator.di

@@ -0,0 +1,11 @@
+// D import file generated from 'source\ecs\entity_allocator.d'
+module ecs.entity_allocator;
+import ecs.manager;
+import std.experimental.allocator;
+import std.experimental.allocator.mallocator : AlignedMallocator, Mallocator;
+struct EntityAllocator
+{
+	void* next_block;
+	void* getBlock();
+	private void allocBlock();
+}