newtype structs and tuple structs should have (or just allow) priv fields

[thestinger]

At the moment a newtype struct can just be constructed and used as the original type with a dereference outside of the module. It's not possible to use them as an independent new type.

[alexcrichton]

Nominating for feature-complete

[graydon]

closing INVALID, we don't have privacy on tuples; make a field name if you want privacy