More generics

[Joannis]

Currently, many functions require an array or other specific collection type where it's not necessary.

An example: public static func validateDatabaseIntegrity(problemLimit limit: Int = Int.max, types: [_Model.Type]) throws -> [DatabaseProblem] where types could easily be a Set, AnySequence or any other sequence of _Model.Type.

In addition to this, _Model.Type... might make the API more easy, too