This adds the simplest possible regression test to corrode which only runs a c file through gcc as well as corrode + rustc and then runs both applications checking that their exit status is the same.

Moves all logic except a thin wrapper out of the executable Main module and into the library itself which makes it possible for the test module to run corrode directly instead of going through an external process. (Running corrode as an external process does not work at all from the tests since there is no way of knowing where the corrode executable is placed).

… used

I think this is the failing test but it is hard to be certain due to the interleaved output

… working

Turns out that a missing return in the main function is only specified in c99.

At least the mingw standard library uses this and gcc seems to ignore it so it should't be a problem to ignore this in corrode as well