Consider `is` operator to check variant of datatypes

[marc0olo]

as discussed in chat.

given datatype metadata = MetadataIdentifier(string) | MetadataMap(map(string, string)) it could be helpful to easily check if a value is of variant MetadataIdentifier or MetadataMap

[marc0olo]

the discussion popped up when implementing these lines of code which could be replaced with a require then:

• https://github.com/aeternity/aex141-nft-collection-example/blob/e4d8190519d8a2ac3381f0ecd753b3716a030ce1/contracts/CollectionTemplateEditionNFTs.aes#L174