this is allow custom allocator for obj-api structs/tables. (#4520)

added "native_custom_alloc" attribute to tables/structs, eg.

table parent_table( native_custom_alloc:"custom_alloc_name" ) {
...
}

with a custom allocator defined as

template <typename T> class custom_alloc_name : public std::allocator<T> {
public:

 typedef T*       pointer;

 template <class U>
 struct rebind {
  typedef custom_alloc_name<U> other;
 };

 pointer allocate(const std::size_t n) {
      return ....;
 }

 void deallocate(T* ptr, std::size_t n) {
    ...
 }

 custom_alloc_name() throw() {}
 template <class U> custom_alloc_name(const custom_alloc_name<U>&) throw() {}
};
};