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

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 https://github.com/0xMiden/protocol/pull/2437#discussion_r2862039155_