C API cleanup

...which brings some API breakage :(

notable things:

* C and C++ names for structs and variables are now in sync
* common.hpp is much easier to read
* added "trcked by" "tracks ..." to know that some things
  are to be kept in synch between C and C++
* "RaptorQ_Dec_Result" name was too similar to
  "RaptorQ_Decoder_Result". The C++ version calls it
  "Decoder_wait_res", let's go with that, at least it
  is more different and actually more to the point.

Signed-off-by: Luca Fulchir <luker@fenrirproject.org>