Add String concactenation support in messages

[Iltotore]

Describe the solution you'd like Add compile-time String concactenation support in messages. Currently, constraits's message with concactenated strings can only be evaluated at runtime (gives the <Unknown> message at compile-time instead).

Describe alternatives you've considered A potential solution would be to create our own instance of FromExpr[String] based on the stdlib's one with this new case:

• Concactenated values can be evaluated at compile-time if all values can be recovered using another FromExpr[T]