Trying to eliminate the magic:
* Merged two `.out` methods into one;
* Merged `RetrieveOneOutEdge` and `RetrieveManyOutEdge` into `RetrieveOutEdge`;
* Moved `unsafeRetrieve{One, Many}OutEdge` implicits to `AnyTVertex` trait;
* Removed type annotations in the godsImplementation;
* `Singleton` and intermediate values are essential for type inference;