Move filewriter.readFile and filewriter.writeFile to util module.
These two functions do not refer to the rest of the filewriter module,
and through moving them the runexecutor needs one dependency less
when used in stand-alone mode.

git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@15021 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c