imperator: unhardcode map files' paths, expand default.map validation

[IhateTrains]

• Tiger no longer assumes the provinces.png, definition.csv and adjacencies.csv file names, it now gets the file names from default.map instead.
• Unexpected entries in default.map are reported now (closes a TODO).
• Removed a TODO from triggers.rs after trying out a binary search implementation. In a benchmark it was slower than the current one.