Test storage key decoding or ask Parity for advice

[daniel-savu]

Is your feature request related to a problem? Please describe.
The decoding of storage keys used in the clients will break if the hasher used by their containing map changes (e.g. switching to BLAKE_2_128 from TWOX_64). That is because the decoding uses hardcoded assumptions, in spite of the hasher info being available in the metadata.

Describe the solution you'd like
Either add tests so the pipeline breaks on such a change, or ask Parity if there is an automatic way of using metadata information to decode storage keys.

[gregdhill]

The metadata does contain the hashers: https://github.com/paritytech/frame-metadata/blob/0c6400964fe600ea07f8233810415f6958fe4e20/frame-metadata/src/v14.rs#L287