Fix Bugs

* Metadata: [BUG FIX] To perform correctly against [Issue #13](https://github.com/urasandesu/Prig/issues/13) at Prig, change the determination process to not only use the original `AssemblyMetadata` equivalence but also the saved `AssemblyGenerator` equivalence when resolving `AssemblyGenerator` from `AssemblyMetadata`. At the same time, deleted the parts controlling them by some flags. On the whole, changed the setters that take the original `.*Metadata` to extract the original `.*Metadata` recursively if the passed value is `.*Generator`.