Add a C version of bob-walker

Claims that the Go version of bob-walker is fast can now be tested, by comparing to this C implementation. To run through the 3 word list 1 million times in C takes 0.31 seconds. With the 5 word list in sport mode, this takes only 0.51 seconds.

Beat that Hipsters!

(Yes, I'm aware that this is probably extremely bad C, but this has never stopped me writing code before, nor will it stop me in future)