Implement proper `Display` for fungible and non-fungible assets

[PhilippGackstatter]

Fungible asset and non-fungible asset's Display impl should be adapted. It currently delegates to the Debug implementation which is not a stable representation (e.g. it could change with non-breaking changes) and Debug shouldn't be used for user-facing error messages. We may just want to use the hex representation of the underlying key and value words, though maybe there is a better idea.

Updating with alternative:

I'd probably go with a more user-readable representation here - e.g., consisting of issuer and amount.

Originally posted by @bobbinth in #2437 (comment)

[PercyDikec]

I've opened a PR to address this I hope all okay :)
PR -> #2550

The Display impl for FungibleAsset now shows issuer and amount, and NonFungibleAsset shows issuer and value elements as hex, instead of delegating to Debug.

@bobbinth @PhilippGackstatter would appreciate your review.