Change private structs to have private fields by default

Change private structs to have private fields by default

This was the original intention of the privacy of structs, and it was
erroneously implemented before. A pub struct will now have default-pub fields,
and a non-pub struct will have default-priv fields. This essentially brings
struct fields in line with enum variants in terms of inheriting visibility.

As usual, extraneous modifiers to visibility are disallowed depend on the case
that you're dealing with.

Closes #11522