Derive field publicity based on source struct fields in derive macro

Right now we derive:
```rust
pub struct Foo {
  a: u32,
  pub(crate) b: u32,
}

// => 
pub struct FooRef<'a> {
  pub a: &'a u32,
  pub b: &'a u32,
}
```

We should probably rather derive the field publicity based on the source struct, so in this case `b` would become `pub(crate)` while `a` is entirely private.