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() {}
};
};

docs/source/CppUsage.md

@@ -126,6 +126,45 @@ The following attributes are specific to the object-based API code generation:
     "native_inline", the value specified with this attribute will be included
     verbatim in the class constructor initializer list for this member.
 
+-   `native_custom_alloc`:"custom_allocator" (on a table or struct): When using the
+    object-based API all generated NativeTables that  are allocated when unpacking 
+    your  flatbuffer will use "custom allocator". The allocator is also used by 
+    any std::vector that appears in a table defined with `native_custom_alloc`. 
+    This can be  used to provide allocation from a pool for example, for faster 
+    unpacking when using the object-based API.
+
+    Minimal Example:
+
+    schema:
+
+    table mytable(native_custom_alloc:"custom_allocator") {
+      ...
+    }
+
+    with custom_allocator defined before flatbuffers.h is included, as:
+
+    template <typename T> struct custom_allocator : public std::allocator<T> {
+
+      typedef T *pointer;
+
+      template <class U>
+      struct rebind { 
+        typedef custom_allocator<U> other; 
+      };
+
+      pointer allocate(const std::size_t n) {
+        return std::allocator<T>::allocate(n);
+      }
+
+      void deallocate(T* ptr, std::size_t n) {
+        return std::allocator<T>::deallocate(ptr,n);
+      }
+
+      custom_allocator() throw() {}
+      template <class U> 
+      custom_allocator(const custom_allocator<U>&) throw() {}
+    };
+
 -   `native_type`' "type" (on a struct): In some cases, a more optimal C++ data
     type exists for a given struct.  For example, the following schema: