zero copy api

[Karsten1987]

connects to ros2/ros2#785
Signed-off-by: Karsten Knese karsten@openrobotics.org

[dirk-thomas]

How does the loaning apply to a dynamic size message like String?

[Karsten1987]

We should highlight this a little bit stronger in the design doc, but essentially it's the middleware's task to declare whether or not it can loan messages of a certain message type.

If yes (for example for fixed sized arrays or POD), the memory for the loaned message is allocated by the middleware. If not, the loaned message is using an alternatively provided instance of an allocator. The idea is that if the middleware doesn't support message loaning, the user should be able to control the allocation.

The logic is implemented here: https://github.com/ros2/rclcpp/pull/864/files#diff-8c68c968941e2e05ba8974455ea3f1b5R73-R74

Independent of the middleware, the user code should always work (whether it is zero-copy or not).

[jacobperron]

LGTM