Make generated C code and icode simpler

[JukkaL]

Currently the generated C code is very verbose. It should be simpler. This would make the code easier to read and debug.

Here are some ideas:
- Add single icode opcode for return None.
- Remove unnecessary gotos to the next label.
- Use macros for common int operations (inline functions might degrade efficiency).
- Use macros/inline functions for calls with 1..4 arguments, for example.
- Simplify function prolog code.

[JukkaL]

Closing this as the C back end is no longer being developed.