message: add a way to import/export an Entity from/to an integer
  This is intended to be used for interop with other programing languages.
diff: #define TNT_UTILS_ENTITY_H
  // FIXME: could we get rid of <functional>
  -#include <functional>
  +#include <functional> // for std::hash
  #include <stdint.h>
  #include <stddef.h>
  @@ -47,6 +47,7 @@ public:
  return mIdentity == 0;
  }
  + // an id that can be used for debugging/printing
  uint32_t getId() const noexcept {
  return mIdentity;
  }
  @@ -55,6 +56,16 @@ public:
  void clear() noexcept { mIdentity = 0; }
  + // Exports an entity to an int32_t which can be used "as is" in the Java programing language.
  + static int32_t smuggle(Entity entity) noexcept {
  + return int32_t(entity.getId());
  + }
  +
  + // Imports an entity from an int32_t generated by smuggle() above.
  + static Entity import(int32_t identity) noexcept {
  + return Entity{ Type(identity) };
  + }
  +
  private:
  friend class EntityManager;
  friend class EntityManagerImpl;
  