typesupport c reloaded

[dirk-thomas]

Similar to the recent changes to the C++ typesupport this PR together with the many referenced ones applies similar changes to the C typesupport:

• all users of the typesupport always use rosidl_typesupport_c
• rosidl_typesupport_c stores a dictionary of available typesupports
• the rmw implementation use a function from the typesupport struct to extract the specific typesupport they support

Linux () and Mac OS () are looking good. I am still looking into Windows.

But while I am doing that I am already asking for review of this and all referenced PRs (either comment on all linked PRs individually or just here with a note that it covers all PRs).

See ros2/ros2#302

[dirk-thomas]

Finally Windows "agrees" too:

• 
• 
• 

[dirk-thomas]

I will merge this in order to continue on the typesupport refactoring. If anyone has comments I am happy to address them after the fact.