+run_phase :: Phase
+ -> String -- basename of original input source
+ -> String -- its extension
+ -> FilePath -- name of file which contains the input to this phase.
+ -> FilePath -- where to stick the result.
+ -> IO (Maybe FilePath)
+ -- Nothing => stop the compilation pipeline
+ -- Just fn => the result of this phase can be found in 'fn'
+ -- (this can either be 'input_fn' or 'output_fn').