Support returning multiple validation errors

[fenollp]

Today, validation ends on the first validation error encountered.

This asks that all validation errors be aggregated in some kind of collection, so all validation errors (or at least, as many as possible or that makes sense) are returned.

This can be behind a generation flag so both behaviors can be generated. Generating both at once (so both ways are available at runtime) should probably not be a goal.

[Adphi]

Maybe we could add a second method validate_all to the Validator trait.

[Adphi]

@fenollp It shouldn't be too hard to implement.
Do you want to give it a go?