Draft of the Simple Message Assigned Message Identifiers REP

This PR proposes a REP that would document all 'assigned' message type identifiers as used in `simple_message`. I think it is important to manage the use of these identifiers centrally, as ad-hoc usage of `msg_type` values could lead to really nasty problems.

Link to the rendered document: [link](https://github.com/gavanderhoorn/rep-ros-i/blob/rep-xxxx_assigned_msg_ids/rep-ixxxx.rst).

Todo:
 - the REP should probably reserve a range as *freely usable*, similar to the *Vendor specific* ranges.
 - I'm not too sure about the *application procedure*: I feel it is important to formalise it a bit, but it should also not be too formal / rigid. I think an approach similar to how REPs in general are reviewed and voted on would be a good compromise.
 - the `msg_type` field is currently a `shared_int`, and is (de)serialised by classes in `simple_message` as a signed integer, which would make negative IDs valid values for the field. I assume that is not really a design choice, but more a consequence of how the *shared types* system was set up? If it is intentional, the negative IDs should be listed in the table as well.