Make small records of struct type

[xperiandri]

Records EntityIdentifier, OperationResult, EntityMetadata have only 2 fields which means that making them structs is efficient option.

[daniel-chambers]

Pretty sure when I started this library, you couldn't make records value types. I'm not fundamentally opposed to the idea, but it would be a gentle breaking change.

Did you have some specific performance issues with them as reference types?