Default __toString() implementations for ValueObjectTrait, SingleValueObjectTrait and EnumTrait

[lhellemons]

It would be nice if value objects and enums could be outfitted with a 'default' __toString() implementation that returns a meaningful expression of their value.

Suggested format:

• for EnumTrait: simply the instance ID.
• for SingleValueObjectTrait: simply the serialized value.
• for ValueObjectTrait: a concatenation of the stringified constructor arguments, similar to the default toString implementation for data classes in many languages