Core functionality to shorten substrate metadata and provide proving mechanism in accordance with RFC0046

Supported are metadata version RuntimeMetadataV15 and above.

RuntimeMetadataV14, while containing similarly structured types registry, is not supported, because the set of types itself is different, with RuntimeMetadataV14 having types not available in RuntimeMetadataV15 and vise versa. Additionally, extension types from SignedExtensionMetadata are referred through different id's in V14 and V15. While V14 and V15 could be both fetched from the node during the transitioning phase, supporting both would thus not be feasible. As V14 is becoming obsolete, it was decided to drop it altogether. Further versions (above V15) will retain compatibility.