Implement a caching system for gitHashes of manifest subtrees.

Also avoid providing a method to Pathmap that would crash if called on non-manifests, by using a new Manifest structure that wraps a PathMap for type safety.

Some tests should be written to check that the caching system really works and avoids computing hashes anew in bigger repositories. Maybe the test should be created before, without caching, and when it is known bigger repositories compute their hash correctly switch on the cache. Or maybe you'd prefer a runtime option to switch the cache on/off.