Modified to use void instead of empty tuples to represent 0 keyword parameters.
Ensure file resolver does not return null, which can happen when it attempts to access an inaccessible file (e.g., because of security permissions)
Many checker updates, especially focused on serializing configurations and using serialized results
Commented out test that is not relevant with modified checker
Merge branch 'master' of github.com:cwi-swat/rascal
Switched keyword params back to value types so they can be serialized