add C bindings

[baggepinnen]

WIP solution along the lines of "solution 2" from #1

Current questions:

• There appears to be two different ways of using the C interface, one called the "casadi way" and one slightly lower level. It's not quite obvious to me which functions I need to provide and to call should I pick one or the other.
• The concept of a "capsule" appears important but does not appear in the documentation.
• To create a plan and ocp_nlp_config etc. I need to call functions that aren't included in the "interface" header files. It thus feels like I shouldn't have to create those objects, but I can't quite tell.