REP - Interface Definition and Language Mapping

[ijnek]

I believe the ROS 2 design article Interface Definition and Language Mapping should be formalized as a REP.

The REP should cover:

• Supported subset of IDL
• Mappings between IDL, ROSIDL, C, C++, Python for all data types available

The ROS 2 documentation has some outdated (incorrect) information (ros2/ros2_documentation#2884), and while I was updating this I figured that the details should be a REP rather than be in the docs.

This is my first time contributing a REP, so just wanted to make sure this is something we'd want before I start working on a PR. Also, how should I number the rep?