This is a meta ticket to collect some related tasks that have been targeted for future releases and that are related to memory management within ROS 2 and it's various layers.

• Expand usage of rcutils_allocator_t
  • rmw: all rmw functions which may allocate memory internally need to take an allocator so it can be passed down from the rcl functions which call it
  • rosidl for C types: all code generators (including rosidl_generator_c as well as middleware specific code generators) should be updated to take allocators where it makes sense (e.g. array initialization functions)
  • rosidl for C++ types: the std::allocator pattern is already in use, but it should be audited to make sure it's being used correctly
• Provide mechanism to use memory allocated by the middleware (get a message loan from the middleware, fill it with data, use it to publish, return the loan)
  • API for pre-allocating messages in the middleware rmw#159
• Remove unnecessary copies and memory allocations in publish/take calls (more generally in calls made in "steady state" of typical nodes)
• Audit documentation for declarations about dynamic memory usage
  • rcutils
  • rmw
  • rosidl
  • rcl