permit impls to map to private fields

[nikomatsakis]

The current RFC bars mapping public trait members to private fields. @petrochenkov questioned this, and I think rightly so. Basically, there is no major difference between:

impl Foo for Bar {
    f = self.g
}

and

impl Foo for Bar {
    fn f(&self) -> &G { &self.g }
}

[crumblingstatue]

Allowing private fields is very useful for use in the implementation of provided methods, but it could lead to unintentionally exposing private fields to users, potentially breaking invariants.

I believe this is a strong motivation for Private trait items.